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University of Windsor University of Windsor
Scholarship at UWindsor Scholarship at UWindsor
Electronic Theses and Dissertations Theses, Dissertations, and Major Papers
2013
Development of a New Forming Process to Fabricate a Wide Development of a New Forming Process to Fabricate a Wide
Range of Phantoms that Highly Match the Acoustical Properties Range of Phantoms that Highly Match the Acoustical Properties
of Human Bone of Human Bone
Adrian Wydra University of Windsor
Follow this and additional works at: https://scholar.uwindsor.ca/etd
Recommended Citation Recommended Citation Wydra, Adrian, "Development of a New Forming Process to Fabricate a Wide Range of Phantoms that Highly Match the Acoustical Properties of Human Bone" (2013). Electronic Theses and Dissertations. 4937. https://scholar.uwindsor.ca/etd/4937
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Development of a New Forming Process to Fabricate a Wide Range of Phantoms that Highly Match the Acoustical Properties of
Human Bone
By
Adrian Wydra
A Thesis Submitted to the Faculty of Graduate Studies
through the Department of Physics in Partial Fulfillment of the Requirements for
the Degree of Master of Science at the University of Windsor
Windsor, Ontario, Canada
2013
© 2013 Adrian Wydra
Development of a New Forming Process to Fabricate a Wide Range of Phantoms that Highly Match the Acoustical Properties of
Human Bone
by
Adrian Wydra
APPROVED BY:
______________________________________________ Dr. J. Sokolowski
Department of Mechanical, Automotive and Materials Engineering
______________________________________________ Dr. W. Kedzierski
Department of Physics
______________________________________________ Dr. R.Gr. Maev, Advisor Department of Physics
June 24, 2013
iii
Declaration of Co-Authorship / Previous Publication
I. Co-Authorship Declaration I hereby declare that this thesis incorporates material that is result of joint research, as follows: This thesis incorporates the outcome of research under the supervision of Professor Roman Maev. The previously published research is covered in Chapter 3 of the thesis. This thesis also incorporates the outcome of a joint research undertaken in collaboration with Eugene Malyarenko and Kiyanoosh Shapoori under the supervision of Professor Roman Maev. The collaboration is covered in Chapter 6 of the thesis. In all cases, the key ideas, primary contributions, experimental designs, data analysis and interpretation, were performed by the author, and the contribution of co-authors was primarily through the provision of advice and guidance. Dr. Eugene Malyarenko also assisted with some of the writing and providing a computer program written in LabView. Kiyanoosh Shapoori helped with an adaptation of the invented method into a phased array system. I am aware of the University of Windsor Senate Policy on Authorship and I certify that I have properly acknowledged the contribution of other researchers to my thesis, and have obtained written permission from each of the co-author(s) to include the above material(s) in my thesis. I certify that, with the above qualification, this thesis, and the research to which it refers, is the product of my own work. II. Declaration of Previous Publication This thesis includes 2 original papers that have been previously published/submitted for publication in peer reviewed journals, as follows: Thesis Chapter Publication title/full citation Publication status Chapter 3 A novel composite material specifically
developed for ultrasound bone phantoms: cortical, trabecular and skull, A. Wydra and R. Gr. Maev. Phys. Med. Biol (submitted in May 2013)
Submitted
Chapter 6 Development of a practical ultrasonic approach for simultaneous measurement of the thickness and the sound speed in human skull bones: a laboratory phantom study, A Wydra, E Malyarenko, K Shapoori and R Gr Maev. Phys. Med. Biol. 58 (2013) 1083–1102
Published
I certify that I have obtained a written permission from the copyright owner(s) to include the above published material(s) in my thesis. I certify that the above material describes work completed during my registration as graduate student at the University of Windsor.
iv
I declare that, to the best of my knowledge, my thesis does not infringe upon anyone’s copyright nor violate any proprietary rights and that any ideas, techniques, quotations, or any other material from the work of other people included in my thesis, published or otherwise, are fully acknowledged in accordance with the standard referencing practices. Furthermore, to the extent that I have included copyrighted material that surpasses the bounds of fair dealing within the meaning of the Canada Copyright Act, I certify that I have obtained a written permission from the copyright owner(s) to include such material(s) in my thesis. I declare that this is a true copy of my thesis, including any final revisions, as approved by my thesis committee and the Graduate Studies office, and that this thesis has not been submitted for a higher degree to any other University or Institution.
v
ABSTRACT
In the various stages of developing diagnostic and therapeutic equipment, the use
of phantoms can play a very important role in improving the process, and help with
implementation, testing and calibrations. However, devices that use different physical
factors, such as MRI, Ultrasound, CT Scan, etc. require the phantom to be made with
different physical properties. This thesis deals with ultrasound and it introduces a novel
composite material and a new forming process to fabricate a wide range of phantoms that
highly match the acoustical properties of human bones. In contrast to ex vivo tissues, the
proposed material can maintain its custom designed physical and acoustical properties
unchanged for long periods of time. As results, the author introduces examples of already
manufactured ultrasound phantoms (i.e. human head phantom) and a novel method of
simultaneous measurements of skull thickness and its sound velocity using a set of skull
bone phantoms.
vi
DEDICATION
I lovingly dedicate this thesis to my wife who has been a great source of
motivation and inspiration.
Also, this is dedicated to my parents for their support all the way since the
beginning of my studies.
Finally, this thesis is dedicated to all those who believes that dreams may come
true.
vii
ACKNOWLEDGEMENTS
Foremost, I would like to express my sincere gratitude to my advisor, Dr. Roman
Gr. Maev, for his continuous support through the process of my Master study and
research, for his patient, guidance, encouragement and providing me the perfect
atmosphere and excellent research team members to work with.
I would like to thank Dr. Mircea Pantea and PhD candidate Kiyanoosh Shapoori,
and all my colleagues who work for the Institute for Diagnostic Imaging Research
(Windsor, Ontario, Canada). I would like to also thank Dr. Eugene Malyarenko from
Tessonics Corp. (Birmingham, Michigan, USA) for his valuable advice and help.
I would like to express thanks to the Ontario Brain Institute (OBI) and to the
Federal Economic Development Agency for Southern Ontario (FedDev Ontario) for their
immense financial support of the research.
Last but not the least, I am heartily thankful to my wife and my parents who have
been a constant source of encouragement and reassurance.
viii
TABLE OF CONTENTS
Declaration of Co-Authorship / Previous Publication ......................................................... iii ABSTRACT ..........................................................................................................................v DEDICATION ..................................................................................................................... vi ACKNOWLEDGEMENTS ................................................................................................ vii LIST OF TABLES ............................................................................................................... xi LIST OF FIGURES ............................................................................................................ xiii LIST OF ABBREVIATIONS/SYMBOLS ........................................................................ xvii
CHAPTER 1 Introduction ................................................................................................... 1
1.1. Why Do We Need Phantoms? .......................................................................................... 1
1.2. Review of Ultrasound Phantoms Manufacturers ............................................................. 2
1.3. Ultrasound Phantoms Available in the Market ................................................................ 4
1.4. Conclusions ....................................................................................................................... 9
1.5. References ........................................................................................................................ 9
CHAPTER 2 Anatomy and Properties of Bones ............................................................... 10
2.1. Bone Structure ................................................................................................................ 10
2.1.1. Cortical Bone ......................................................................................................... 11
2.1.2. Trabecular Bone .................................................................................................... 12
2.1.3. Calcaneus ‐ Heel Bone ........................................................................................... 13
2.1.4. Skull Bone .............................................................................................................. 14
2.2. Mechanical and Acoustical Properties of Bones ............................................................. 16
2.2.1. Mechanical Properties .......................................................................................... 16
2.2.2. Sound Velocity ....................................................................................................... 17
2.2.3. Ultrasonic Attenuation .......................................................................................... 20
2.2.4. QUS ‐ Quantitative Ultrasound ............................................................................. 22
2.3. References ...................................................................................................................... 24
CHAPTER 3 A Novel Composite Material Specifically Developed for Ultrasound Bone Phantoms: Cortical, Trabecular and Skull ......................................................................... 27
3.1. Introduction .................................................................................................................... 27
ix
3.2. Material: Developing Process and Method of Manufacturing ....................................... 28
3.3. Methods of Investigation of a Developed Material ........................................................ 34
3.4. Properties of the Material ‐ Results ................................................................................ 39
3.4.1. Flat Non‐porous Bone Phantoms .......................................................................... 39
3.4.2. Various Phantoms and their Properties ................................................................ 42
3.5. Discussion ....................................................................................................................... 48
3.6. Conclusions ..................................................................................................................... 49
3.7. References ...................................................................................................................... 50
CHAPTER 4 A Theoretical Prediction of Acoustical Properties of the Developed Composite Material .......................................................................................................... 53
4.1. Reuss’ and Voigt’s Model ................................................................................................ 53
4.2. Average T‐Matrix Approximation (ATA) ......................................................................... 55
4.3. Coherent Potential Approximation (CPA) ....................................................................... 55
4.4. Devaney Model ............................................................................................................... 56
4.5. Summary ......................................................................................................................... 57
4.6. References ...................................................................................................................... 59
CHAPTER 5 Examples of Manufactured Bone Phantoms and Their Applications .......... 60
5.1. Ultrasound Skull Phantom .............................................................................................. 60
5.2. Soft Tissue Phantom ‐ An Artificial Brain ........................................................................ 64
5.3. Ultrasound Head Phantom ............................................................................................. 69
CHAPTER 6 Development of a Practical Ultrasonic Approach for Simultaneous Measurement of the Thickness and the Sound Speed in Human Skull Bones: a Laboratory Phantom Study ............................................................................................... 70
6.1. Introduction .................................................................................................................... 70
6.2. Phantoms and Methods of Their Investigation .............................................................. 73
6.3. Results from the Experimental Work ............................................................................. 85
6.3.1. Measurements Using a Single‐Element Focused Transducer ............................... 85
6.3.2. Measurements Using a Linear Phased Array Probe .............................................. 91
6.4. Discussion ....................................................................................................................... 93
6.5. Conclusions ..................................................................................................................... 96
6.6. References ...................................................................................................................... 96
CHAPTER 7 Final Conclusions and Future Work ............................................................. 99
x
7.1. Final Conclusions ............................................................................................................ 99
7.2. Future Work .................................................................................................................. 100
FULL REFERENCE LIST ...................................................................................................... 103 APPENDIX A .................................................................................................................. 111
Copyright permissions ......................................................................................................... 111
VITA AUCTORIS ............................................................................................................... 120
xi
LIST OF TABLES
Table 2.1: Summary statistics for diploeic thickness measured by sex and location (Lynnerup et al 2005). ............................................................................................................................................. 15
Table 2.2: Statistics for cranial thickness measured by sex and location (Lynnerup 2001). ......... 16
Table 2.3: Mechanical properties of human bones (Teoh and Chui, 2008). .................................. 17
Table 2.4: Summary of the ultrasonic wave velocity with the frequency for different types of human bone tissue and few selected animals (Janson et al 1990). ................................................ 20
Table 2.5: Review of the ultrasonic parameters of bone basis on a few literature sources. ........... 23
Table 3.1: Summarized properties of the ingredients used for the new material. .......................... 30
Table 3.2: The properties for the trabecular bone phantoms. ........................................................ 43
Table 3.3: Ultrasonic properties of the flat and flat-wrinkled phantoms with and without porosity layer (diploe). ................................................................................................................................. 46
Table 4.1: Mechanical properties of components used for the developed composite material for ultrasound bone phantoms. ............................................................................................................ 54
Table 5.1: Properties of brain and tissue mimicking equivalent used for brain phantom. ............. 65
Table 6.1: Properties of the developed skull bone phantom. ......................................................... 75
Table 6.2: Properties of the focused single element transducer used in the performed experiment. ....................................................................................................................................................... 78
Table 6.3: The thickness of the flat thick phantom calculated from the double focused experimental data and its comparison with the thickness measured by caliper. ............................ 86
Table 6.4: The speed of sound in the flat thick phantom calculated from the double focus experimental data (SOSE) and its comparison with the speed of sound measured by transmission method with prior knowledge of the sample thickness (SOST). .................................................... 87
Table 6.5: The thickness and the sound speed of the flat thin phantom without porosity layer obtained from the experiment (averaged value from 10 points) and from an independent reference method. .......................................................................................................................................... 87
xii
Table 6.6: The thickness and the sound speed of the flat thin phantom with porosity layer obtained from the experiment (averaged value from 10 points) and from an independent reference method. .......................................................................................................................................... 87
Table 6.7: Results obtained from phased array probe and curved 3D skull phantom with inner porosity layer. ................................................................................................................................ 93
xiii
LIST OF FIGURES
Figure 1.1: Lumbar training phantom (CIRS, website: www.cirsinc.com). .................................... 5
Figure 1.2: Quantitative Ultrasound Phantom (CIRS, website: www.cirsinc.com). ....................... 6
Figure 1.3: Transparent Internal jugular Central Line Ultrasound Manikin (Blue Phantom, website: www.bluephantom.com). .................................................................................................. 6
Figure 1.4: Custom made ultrasound head phantom. ....................................................................... 7
Figure 1.5: Dual attenuation quality control tests phantom (Gammex, website: www.gammex.com). ........................................................................................................................ 8
Figure 1.6: Ultrasound torso - medical training platform (SynDaver Labs, website: www.syndaver.com). ....................................................................................................................... 8
Figure 2.1: Hierarchical structural organization of bone (Rho et al 1998). ................................... 10
Figure 2.2: The magnitudes of wavelengths and size of bone features. ........................................ 11
Figure 2.3: Low volume fraction trabecular tissue (pore size: 0.5 - 2 mm) (Fyhrie and Kimura, 1999). ............................................................................................................................................. 13
Figure 2.4: The basic arrangement of the subcortical systems of cancellous bone of calcaneal tuberosity. Long double headed arrows: arch-like anteroposterior longitudinal trajectories. Short arrows: subcortical system of cancellous bone of the calcaneal tuberosity. (a) Cancellous bone systems on X-ray snap of a healthy individual (lateral projection). (b) Sagittally sectioned dry calcaneus in the region of the calcaneal tuberosity. Asterisks: thin superficial cortical bone of the tuberosity. Bar: 4 cm. (c) X-ray snap of the heel region of the same individual as in (a) (axial projection). (d, e) Transversally sectioned dry calcanei in their distal (d) and proximal (e) thirds. Bar: 4 cm (Kachlik et al 2008). ..................................................................................................... 14
Figure 2.5: X-ray of a biopsy. The three bone layers of the cranial vault are indicated (Lynnerup et al 2005). ..................................................................................................................................... 15
Figure 3.1: SEM picture of alumina powder is showing the targeted diameter of particles. ......... 31
Figure 3.2: The material for bone phantom during the preparation process. ................................. 32
xiv
Figure 3.3: Program window for calculation of surface porosity of porous samples. Left window - raw image of sample; right window - image converted to black and white colors; center panel - options and calculated porosity ...................................................................................................... 33
Figure 3.4: The prepared porous phantoms with calculated porosity level: 14%, 19%, 22% and 30% (trabecular bone phantoms). .................................................................................................. 33
Figure 3.5: The 21 prepared for the experimental purposes bone phantoms with different composition and the epoxy sample as a reference. ........................................................................ 34
Figure 3.6: Directions of samples and directions of the propagation of ultrasound waves. .......... 35
Figure 3.7: Experimental set-up used for the investigation of attenuation of the prepared phantoms. ....................................................................................................................................... 37
Figure 3.8: Spectrum of the recorded signal for 1 MHz transducer in water without any sample. 38
Figure 3.9: Spectrum of the recorded signal for 2.25 MHz transducer in water without any sample. ........................................................................................................................................... 38
Figure 3.10: The relationship between velocity and density for the developed material for two frequencies: 1 MHz and 2.25 MHz for the density range 1.10 – 2.22 g/cm3. ................................ 40
Figure 3.11: The relationship between the logarithm of bulk modulus and logarithm of density for the developed composite material for the ultrasonic bone phantoms. ........................................... 41
Figure 3.12: The relationship between ultrasound attenuation and density for the developed material for the density range 1.10 – 2.22 g/cm3. .......................................................................... 42
Figure 3.13: Velocity dependence on porosity for the prepared trabecular bone phantoms. ......... 43
Figure 3.14: Flat phantoms made from the composite material with and without porosity layer.. 45
Figure 3.15: Flat-wrinkled phantoms without (upper) and with (lower) porosity layer. ............... 45
Figure 3.16: User interface of the program written in MATLAB for the calculations of ultrasonic attenuation. ..................................................................................................................................... 46
Figure 3.17: Broadband ultrasound attenuation for flat samples with and without diploe layer (measurements were made by using ultrasonic transducer at nominal frequency 2.25MHz). ....... 47
Figure 3.18: The skull bone phantoms with real profile with and without middle porosity layer (diploe). .......................................................................................................................................... 48
Figure 4.1: Comparison between calculated from the theoretical models and experimentally measured longitudinal sound velocity. .......................................................................................... 58
Figure 5.1: Manufactured calvaria phantom with inner porosity layer. ......................................... 61
xv
Figure 5.2: Porosity zones within manufactured calvaria phantom. .............................................. 62
Figure 5.3: Comparison between actual skull and the manufactured phantom. ............................ 63
Figure 5.4: Completed ultrasound skull phantom made out of developed material. ..................... 64
Figure 5.5: Artificial brain for the ultrasound head phantom. ....................................................... 66
Figure 5.6: Brain phantom within the skull bone phantom and the hematoma phantom attached to the brain surface. ............................................................................................................................ 68
Figure 5.7: Completed full anthropomorphic human head phantom for the transcranial imaging project. ........................................................................................................................................... 69
Figure 6.1: Schematic illustration of the elongated beam and its intensity in the performed experiment. .................................................................................................................................... 73
Figure 6.2: Skull bone phantoms used in the described experiments (upper left – thin flat sample without porosity layer, upper middle – thin flat sample with porosity layer, upper right – thick flat sample, lower left – undulated sample without porosity layer, lower right – undulated sample with porosity). ........................................................................................................................................ 74
Figure 6.3: Skull bone phantom with curved profile and inner porosity layer used for the experiments with a phased array probe. ......................................................................................... 75
Figure 6.4: The principle of the double focus experiment (left – position of the sample and transducer, right – recorded A-Scans from both positions). .......................................................... 78
Figure 6.5: Schematic configuration of the experimental set-up for the single point measurement. ....................................................................................................................................................... 79
Figure 6.6: Recorded A-Scan for the thin flat phantom without porosity. During the double focus experiment the reflections and their amplitudes from the front and back surfaces were tracked (upper graph), and the distance between transducer positions corresponding to the recorded maxima was calculated (lower graph). .......................................................................................... 79
Figure 6.7: Using a flat, linear phased array transducer for measuring the thickness and the sound velocity. .......................................................................................................................................... 83
Figure 6.8: Schematic configuration of the experimental set-up for the phased array transducer. 83
Figure 6.9: The curved porous phantom and the phased array probe during the performed experiment. .................................................................................................................................... 84
Figure 6.10: Time delay patterns for the phased array probe for 7 out of 48 programmed focal points (3mm, 5mm, 7mm and so on). ............................................................................................ 84
Figure 6.11: An example A-Scan recorded on the bone phantom with a single porosity (diploë) layer. .............................................................................................................................................. 86
xvi
Figure 6.12: (a) – Thickness profile measured 4 times for the same sample; (b)– Averaged thickness profile of the undulated sample obtained with the double focus method superimposed on the actual thickness profile; (c) – Ultrasonic B-Scan of the same specimen; (d) – The profile obtained from the B-Scan and the speed of sound measured at a single zero-slope point. ............ 88
Figure 6.13: Reflections of an obliquely incident focused beam at two different transducer positions: more ultrasonic energy is captured by the transducer in the defocused position. ......... 89
Figure 6.14: a) – Ultrasonic B-Scan of the sample with inner porosity layer; b) Ultrasonic B-Scan with the sound speed correction obtained from the experiment and the actual superimposed sample profile in the background. .................................................................................................. 89
Figure 6.15: Example of 6 out of 48 A-Scans obtained with the phased array transducer for the thin flat phantom without porosity layer (each line represents a different focal point: ~3mm, ~5mm, ~7mm and so on). .............................................................................................................. 93
Figure 7.1: Calibration phantom for the transcranial blood flow imaging project ...................... 101
xvii
LIST OF ABBREVIATIONS/SYMBOLS
SOS – Speed of sound
BUA – Broadband ultrasound attenuation
TOF - Time of flight
E - Young's modulus
K - bulk modulus
G - shear modulus
M - elastic modulus
ρ - density
C - longitudinal sound velocity
T - transmission coefficient for pressure
R - reflection coefficient for pressure
α - ultrasound attenuation
Z - acoustic impedance
1
CHAPTER 1 Introduction
The research on the properties of ultrasound and how ultrasound propagate
through the biological medium requires knowledge of specific details of such medium,
and the phantoms can act as a medium in which all the desired ultrasonic parameters can
be mimicked. In the case of bones (as well as for other biological tissues), it is necessary
to use a medium that has values of sound velocity, ultrasonic attenuation and density on a
similar level or, in ideal case - exactly the same as real bones (Wydra and Maev, 2013).
1.1. Why Do We Need Phantoms?
Research on real soft tissues or the human body involves several distinct
problems. First of all, considering ex vivo samples, it is hard to keep their properties
unchanged for long period of time. Especially, regarding to ultrasound, the acoustical
properties of human tissues change very fast. If one wants to keep them unchanged for
longer time, one has to make efforts and introduce chemical substances and/or very low
temperatures. Moreover, the research on real biological tissues always requires
permission from certain ethical institutions and there is always a risk that organic samples
may be already contaminated or they may become contaminated in the future. This can
become seriously dangerous for the researchers and technicians who work with such
samples.
The problem with real tissues appears also when one wants to investigate some
certain type of abnormality or some tissue properties which are not very common. This
requires special samples which may not be available right away.
As a solution, the researchers and technicians can use phantoms which are
specifically designed to mimic certain properties of normal and abnormal tissues. The
phantoms which mimic acoustical properties of living tissues can replace normal tissues
during the process of developing new medical devices and during the learning process for
technicians and students who have to get familiar with the new equipment and
technology. Nowadays, phantoms become an integral part of modern research and can
2
provide repeatable results without dealing with various problems related to actual living
tissues. Additionally, they are stable in time which is particularly important if one wants
to obtain repeatable measurements and get the same result for long period of time.
1.2. Review of Ultrasound Phantoms Manufacturers
In the modern market, one can easily find several companies which are involved
with biomedical phantoms. These companies are focused on manufacturing phantoms for
various applications and selling them, usually for a very large profit. Examples of the
biggest and the most known companies in the world are listed as follows:
• CIRS - Computerized Imaging Reference Systems, Inc. (Norfolk, Virginia, USA)
- The company was established in 1983 and began its work with a clearly defined
mission to improve tissue simulation methodology and provide quantitative
reference standard for Computed Tomography. Today, CIRS is one of the leaders
in manufacturing tissue equivalent phantoms and simulators for medical imaging,
radiation therapy and procedural training. It offers various phantoms with the
possibility of customization for particular research problems (CIRS, website:
www.cirsinc.com).
• Blue Phantom (Redmond, WA, USA) - It is an eleven year old company which is
already becoming one of the world leaders in designing and developing
ultrasound hands-on training models and comprehensive education phantoms. The
company’s goal is to provide very realistic ultrasound phantoms. They currently
do not offer phantoms for any other physical applications. They mainly target
clinicians to help them to better care for patients in their communities (Blue
Phantom, website: www.bluephantom.com).
• Kyoto Kagaku (Kyoto, Japan; Tokyo, Japan; Torrance, CA, USA) - Kyoto
Kagaku is one of the oldest phantom manufacturers in the world. The company
was founded in 1948 but it originates from the Shimadzu Corporation, founded by
Genzo Shimadzu in 1875. The company has been providing to the medical and
educational institutions a wide array of skill training products. Today, they offer a
3
wide range of phantoms such as patient simulators for diagnosis and nurse
training as well as puncture and injection models and imaging phantoms for
ultrasound and radiology exam training (Kyoto Kagaku, website:
www.kyotokagaku.com).
• Gammex (Middleton, WI, USA; Nottingham, United Kingdom; Giessen-
Allendorf, Germany) - Gammex was founded in 1969 by Dr. Charles Lescrenier
and it started from introducing lasers into radiation therapy departments for use in
precision patient alignment. In 1987 the company acquired Radiation
Measurements, Inc and together, and they expanded to manufacture not only
lasers but also quality-control test equipment (phantoms) for diagnostic radiology,
ultrasound, mammography, CT, and therapy. Today, the company work with
manufacturers, hospitals and clinics around the world (Gammex, website:
www.gammex.com).
• SynDaver Labs (Tampa, Florida, USA) - SynDaver is a relatively young company
founded in 2004 to commercialize a novel system of synthetic human body parts
for the medical device industry. The models manufactured by SynDaver Labs
replicate human anatomy including individual muscles, tendons, veins, arteries,
nerves and organs (all made from composite materials that mimic the properties
of discrete living tissues). Moreover, the company holds one of the world's largest
databases of live tissue properties. As an ultimate goal they plan to replace the
live animals and human cadavers in medical education and training with synthetic
analogs which are more cost-effective than their real equivalents (SynDaver,
website: www.syndaver.com).
• ATS Laboratories, Inc. (Bridgeport, CT, USA) - The company was established in
1978 as a privately held Corporation. ATS designs and manufacturers tissue-
mimicking phantoms used to monitor performance changes which may occur
during normal operation of an ultrasound imaging system (ATS Laboratories,
website: www.atslaboratories-phantoms.com).
4
1.3. Ultrasound Phantoms Available in the Market
The previous section presented six world leaders companies who specialize
themselves in phantoms manufacturing for various biomedical applications. This section
will briefly introduce examples of ultrasound phantoms which one can already find in the
modern market. The examples listed below do not show phantoms of single hard or soft
tissues. The phantoms always have to be designed for a particular practical application
and this forces manufacturers to produce not a single tissue phantoms but phantoms of
part of the body or even an entire body. Such phantoms consist of various materials
connected to each other with artificial tissues, which is similar to an actual human body.
The examples are listed as follows:
• Lumbar Training Phantom (CIRS) - The phantom was designed and manufactured
to provide a realistic puncture practice used for fluoroscopic image guidance. It
can be imaged under CT, MR and ultrasound. The lumbar phantom (Figure 1.1)
posses an anthropomorphic L-spine anatomy and a self-sealing puncture
membrane which increase its lifetime. The phantom works well for its application
but the manufacturer does not provide any information about the acoustical
properties of the included bone tissue equivalent. Such information would be
particularly important for the author of the following thesis, but since this data is
not provided and the phantom does need a realistic bone tissue for its application,
it is expected that the acoustical properties of the material used for spine may vary
from the properties of actual bones.
5
Figure 1.1: Lumbar training phantom (CIRS, website: www.cirsinc.com).
• Quantitative Ultrasound Phantom (CIRS) - The phantom was designed and
manufactured in order to calibrate ultrasonic devices for bone assessment and for
investigation of osteoporosis. It mimics acoustical properties of the calcaneus
(heel bone) and can be moulded to any shape. The phantom comes in two
versions (Figure 1.2): the first one has the properties of a healthy bone; the second
one has the properties of the bone affected by osteoporosis. Since the QUS
phantom provides acoustical properties of a calcaneus, it is very important from
the point of view of this thesis. The example shows that phantoms designed for
calibration of ultrasonic bone densitometers exist and they are being sold in the
market. However, the phantoms mimic the sound velocity and ultrasound
attenuation but CIRS does not mention anything about its density and porosity.
This may lead to the conclusion that those two parameters may be different in the
phantom than in an actual calcaneus. In this case, the phantom may be used for its
application (calibration of the medical devices) but it may not be used as material
which mimics all four properties (SOS, BUA, density, porosity) on the same level
as in actual bones.
6
Figure 1.2: Quantitative Ultrasound Phantom (CIRS, website: www.cirsinc.com).
• Transparent Internal Jugular Central Line Ultrasound Manikin (Blue Phantom) - It
is an interesting phantom which was designed to help students and clinicians learn
and be more familiar with the ultrasonic imaging techniques and devices. The
advantage of this model is its transparency, which allows students to see the
investigated object not only on the computer screen but also with the naked eye.
The phantom is shown in Figure 1.3.
Figure 1.3: Transparent Internal jugular Central Line Ultrasound Manikin (Blue Phantom, website: www.bluephantom.com).
• Ultrasound Head Phantom (Kyoto Kagaku) - The phantom was specifically
designed for the ultrasonic transcranial blood flow imaging project which is one
of the high priority projects run in the Institute for Diagnostic Imaging Research
7
(Windsor, Ontario, Canada). The phantom (Figure 1.4) is a result of a
collaboration between the IDIR and the Japanese company Kyoto Kagaku, where
the author was deeply involved in a design and preparation. The head phantom
contains custom made artificial blood vessels, a mechanical pump for blood flow
(made by Kyoto Kagaku) and the bone phantom (made by the author of the
following thesis). The details about the skull bone phantom will be introduced in
CHAPTER 3 and 5.
Figure 1.4: Custom made ultrasound head phantom.
• Dual Attenuation Phantom for Quality Control Tests (Gammex) - Gammex
phantom was created in order to test and calibrate ultrasonic imaging devices. It
contains various sizes of objects located at different depths and it permits quality
control tests over a wide range of frequencies at two different media with two
different attenuations. The phantom (Figure 1.5) is an effective instrument to
demonstrate image quality while challenging high performance ultrasound
systems. It is a very good example of the application when the researcher has to
use an artificial phantom for their set of experiments and the living tissues cannot
provide such results.
8
Figure 1.5: Dual attenuation quality control tests phantom (Gammex, website: www.gammex.com).
• Ultrasound Torso (SynDaver Labs) - According to the manufacturer's website: "it
is a realistic medical training platform designed for users interested in developing
and practicing skills associated with Focused Assessment with Sonography for
Trauma. Anatomic features include: the thoracic and abdominal organs, the aorta,
inferior and superior vena cava, and pericardial fluid" (SynDaver). It is an
interesting phantom which was designed to enhance the psychomotor skills of
surgeons, physicians, paramedics, nurse practitioners, nurses, ultrasound
technicians and physician's assistants. Moreover, the phantom (Figure 1.6) has
realistic artificial tissues that mimic mechanical, thermal and physicochemical
properties of live tissue.
Figure 1.6: Ultrasound torso - medical training platform (SynDaver Labs, website: www.syndaver.com).
9
1.4. Conclusions
All the presented phantoms work well for the applications which they were
designed for, but none of them contain a highly realistic bone tissue equivalent (except
CIRS QUS phantom which mimics only sound velocity and ultrasound attenuation of the
calcaneus). As a response to that, the thesis will introduce a novel composite material that
highly matches all of the acoustical properties of bones: sound velocity, ultrasound
attenuation, density, porosity and acoustic impedance. Moreover, the material can be
successfully used to manufacture any phantoms for various applications, which is the
main goal of this thesis.
1.5. References
ATS Laboratories, website address: http://atslaboratories-phantoms.com/, as of May 3,
2013.
Blue Phantom, website address: http://www.bluephantom.com/, as of May 3, 2013.
CIRS, website address: http://www.cirsinc.com/, as of May 3, 2013.
Gammex, website address: http://www.gammex.com/index.asp, as of May 4, 2013.
Kyoto Kagaku, website address: http://www.kyotokagaku.com/, as of May 4, 2013.
SynDaver, website address: http://www.syndaver.com/, as of May 4, 2013.
Wydra A and Maev RGr. A novel composite material specifically developed for
ultrasound bone phantoms: cortical, trabecular and skull. [Submitted to: Physics in
Medicine and Biology in May 2013].
10
CHAPTER 2 Anatomy and Properties of Bones
The fabrication of bone phantoms can be done only if the properties of actual
bones are known. This requires knowledge about the structure of bones and knowledge
about all of their mechanical and physical properties. The following chapter briefly
introduces the anatomy, morphology and mechano-physical properties of bones.
2.1. Bone Structure
Based on the structure, bone tissue can be divided into cortical and trabecular
bones. Each type of bone has different morphology as well as different mechanical and
acoustical properties. This means that there is a need for a separate analysis of each type
of bone and a need for separate approaches for making distinct phantoms. The
hierarchical structural organization of bone is shown in Figure 2.1 (Rho et al 1998).
Figure 2.1: Hierarchical structural organization of bone (Rho et al 1998).
11
According to the figure, based on the dimensions, bone can be divided into:
macrostructure, microstructure, sub-microstructure, nanostructure and sub-nanostructure.
Such structures must always be considered if one wants to make any bone phantoms.
However, for the phantoms designed for acoustical applications one has to consider only
the features which are comparable in size with the wavelength of sound. For modern
medical devices, the frequency range used for an ultrasonic bone investigation is within
0.5 - 2 MHz. This gives wavelengths in bone matrix: ~3mm at 1MHz and ~1.5mm at 2
MHz. At the same time the diameter of osteon vary from 0.01 to 0.5mm. The numbers
show that the only bone sub-structures that have to be considered for the ultrasound bone
phantoms, are macrostructure and microstructure. The scale which shows the ultrasonic
wavelengths and their corresponding bone features are shown in Figure 2.2.
Figure 2.2: The magnitudes of wavelengths and size of bone features.
2.1.1. Cortical Bone
Cortical bone is a fairly solid and dense material which consists of a mineral part,
organic parts and a water part. The mineral ingredient is hydroxyapatite (~69%); the
12
organic parts are fibrous protein collagen and non-collagenous materials (22%); the water
portion is about 9% of the total mass (Uklejewski 1992, Braidotti et al 1997).
The mineral part is composed of extremely small crystals (4 nm by 50 nm by
50nm) in a variant of hydroxyapatite Ca10(PO4)6(OH)2. The crystals are impure with
about 4-6% of carbonate replacing the phosphate groups.
The organic component is mostly a collagen Type I, but there are small amounts
of Type III and Type VI. One of the interesting things is that a slowly heated collagen
shrinks at a particular temperature, giving an indication of a stability of molecules. The
"male bone" collagen has a shrinkage temperature of about 61.5°-63.5°C up to the age of
about 60 and about 600C over that age. The "female bone" showed much greater
variability. About 10% of the bones organic material is non-collagenous protein (NCP)
(Black and Hastings, 1998).
2.1.2. Trabecular Bone
Trabecular bone consists primarily of lamellar bones which are arranged in
packets that make up an interconnected irregular array of plates and rods, called
trabeculae. The space between trabeculae is filled with bone marrow. Such a structure
makes the trabecular bone a highly heterogeneous material. Most mechanical properties
of trabecular bone depend to a large degree on the apparent density, defined as the
product of the density of the individual trabeculae (the 'tissue density') and the volume
fraction of bone present in the bulk specimen. Black and Hastings (1998) reported that
volume fraction typically ranges from 0.60 for dense trabecular bone to 0.05 for very
porous trabecular bone. The (wet) tissue density for human trabecular bone is fairly
constant and is in the approximate range of 1.6 - 2.0 g/cm3. By contrast, the apparent
density (product of volume fraction and trabecular tissue density) varies substantially and
is typically in the range of 0.05-1.0 g/cm3. The example structure of trabecular bone is
shown in Figure 2.3.
13
Figure 2.3: Low volume fraction trabecular tissue (pore size: 0.5 - 2 mm) (Fyhrie and Kimura, 1999).
2.1.3. Calcaneus - Heel Bone
Human calcaneus belongs to a group of bones which are very easy accessible for
ultrasound. Calcaneus mainly consists of the trabecular type of bone, which is the bone
type mostly affected by bone diseases: including a very serious bone disease -
osteoporosis. An important and interesting feature of the calcaneus is a very thin cortical
bone layer and a thin skin layer. This means that if one wants to in vivo and non-
invasively check the ultrasonic properties of heel bone, one will get the properties mostly
of a trabecular bone structure with very little influence of surrounding tissues. All of
these features make calcaneus one of the most popular tissues for ultrasonic investigation
of osteoporosis. A general view of heel anatomy and its trabecular system is shown in
Figure 2.4.
14
Figure 2.4: The basic arrangement of the subcortical systems of cancellous bone of calcaneal tuberosity. Long double headed arrows: arch-like anteroposterior longitudinal trajectories. Short arrows: subcortical system of cancellous bone of the calcaneal tuberosity. (a) Cancellous bone systems on X-ray snap of a healthy individual (lateral projection). (b) Sagittally sectioned dry
calcaneus in the region of the calcaneal tuberosity. Asterisks: thin superficial cortical bone of the tuberosity. Bar: 4 cm. (c) X-ray snap of the heel region of the same individual as in (a) (axial
projection). (d, e) Transversally sectioned dry calcanei in their distal (d) and proximal (e) thirds. Bar: 4 cm (Kachlik et al 2008).
2.1.4. Skull Bone
Skull bone tissue consists of three bone layers: the inner, the middle and the outer.
The inner and outer layers are cortical bones; the middle layer is a trabecular bone
(Lynnerup et al 2005). Arrangement of different layers is shown in Figure 2.5. The
15
thickness of each layer depends on place of the skull and sex. Table 2.1 shows the
thickness of trabecular bone layer in skull bone for different locations and sex (Lynnerup
et al 2005). Table 2.2 shows the thickness of the entire cranial bone including three bone
layers (Lynnerup 2001).
Figure 2.5: X-ray of a biopsy. The three bone layers of the cranial vault are indicated (Lynnerup et al 2005).
Table 2.1: Summary statistics for diploeic thickness measured by sex and location (Lynnerup et al 2005).
Sampling point Sex n Mean (mm) Std. Dev. (mm)
Frontal Male 41 2.954 1.135 Female 21 2.019 0.966
Occipital Male 37 3.573 1.462 Female 18 2.972 1.476
Right euryon Male 42 1.838 1.128 Female 18 1.961 1.123
Left euryon Male 41 1.724 1.162 Female 19 1.537 1.008
16
Table 2.2: Statistics for cranial thickness measured by sex and location (Lynnerup 2001).
Sampling point Sex Mean (mm) S.D. (mm) Range (mm)
Frontal Male 7.044 1.273 4.970 – 10.630 Female 6.678 1.123 4.490 – 9.340
Occipital Male 7.825 1.657 4.720 – 11.260 Female 7.603 2.013 4.570 – 12.740
Right euryon Male 5.040 1.250 2.980 – 8.590 Female 6.635 1.138 3.160 – 7.880
Left euryon Male 5.034 1.328 2.740 – 8.690 Female 5.452 1.419 3.190 – 8.570
2.2. Mechanical and Acoustical Properties of Bones
From the mechanical point of view, both types of bones (cortical and trabecular)
are porous composite materials. However, the cortical and trabecular bones are
anatomically and structurally different and they also have different mechano-physical
properties. Langton and Njeh (2007) observed that the cortical bone is predominantly
solid and makes up shafts of the long bone in the skeleton. Cancellous bone
predominantly acts as a shock absorber and it is found near the joint surfaces of the long
bones and within the individual vertebrae making up the spinal column.
2.2.1. Mechanical Properties
The values of mechanical properties of bones reported in literatures always have a
very broad range. Depending on location of the human body, bones have to fulfill various
functions and this always corresponds to different properties. Moreover, bones are
anisotropic materials which means that properties are different along different directions.
Thus, the mechanical properties of bone depend on factors such as trabecular orientation,
microstructure and density.
It is known that bone strength follow the Wolff’s law which says that bone adapts
in response to stress and bone structure is arranged in such a way that it optimally bears
physiological loads. This means that bone forms in parallel plates aligned in the direction
of principal load. This is true and it explains the inner bone structure and its orientation.
17
However, for the general simplification, one can assumes with a pretty good
approximation that bone density is the main parameter which determines bone strength;
in general larger density means larger mechanical properties and stronger bone. For the
reference, the averaged values of mechanical properties of bones are collected in Table
2.3.
Table 2.3: Mechanical properties of human bones (Teoh and Chui, 2008).
Type of parameter Type of bone cortical trabecular
Young modulus [GPa] 14 - 20 0.05 - 0.5 Porosity [%] 5 - 30 30 - 90
Poisson’s ratio 0.2 - 0.5 0.01 - 0.35 Density [g/cm3] 1.8 - 2.2 0.3 - 1.3
Compressive strength [MPa] 170 - 193 7 - 10 Tensile strength [MPa] 50 - 150 10 - 20
2.2.2. Sound Velocity
Speed of sound (SOS) in any material depends on structural properties of such
material. In general, two distinct wave velocities can be defined, namely phase velocity
and group velocity. Phase velocity refers to the velocity of a continuous wave (single
frequency); group velocity refers to the velocity of a pulse (wave packet) which contains
a broadband range of frequencies (Pain, 1985). Since bone is known as a dispersive
medium, these two velocities are not the same and the cancellous bone is in a magnitude
more dispersive than the cortical one. The group velocity can be measured
experimentally and from the definition it is given by a well known formula cited by
Langton and Njeh (2007):
td
VelocitySound = (2.1)
where d – sample thickness, t - transit time in sample.
18
In some cases, a laboratory setup may require a water immersed sample between two
ultrasonic transducers (transmitter and receiver). For such a setup, the sound velocity is
given by (Langton and Njeh, 2007):
( )w
w
tCddC
VelocityTOFΔ−
= (2.2)
where TOF – Time of flight, wC - velocity of ultrasound in water, tΔ is the difference in
transit time with and without the sample.
Considering the physics behind sound velocity, it should be noted that sound
velocity is related to Young's modulus and to density and can be expressed by the
following formula cited by Langton and Njeh (2007):
ρECL = (2.3)
Eq.2.3 shows only a general relationship and does not strictly apply to an
anisotropic, heterogeneous and dispersive medium like a bone. In such a case, it is
necessary to consider each direction of sample separately and apply to it an advanced
equation for a longitudinal wave velocity which is as follow (Nguyen and Lethiecq
1996):
( )ρ
GKCL34+
= (2.4)
where K is a bulk modulus and G is a shear modulus.
In general one can express the formula for longitudinal sound velocity as (Frederick,
1965):
( )( )( ) ρρ
Mvv
vECL =−+
−=
2111 (2.5)
where v is a Poisson’s ratio and M is an elastic modulus which is not the same as the
Young’s modulus.
19
At the same time shear velocity can be expressed as follow (Nguyen et al 1996):
ρGCL = (2.6)
It should be noted that sound velocity in bones can dramatically vary within a
wide range and it is dependent on various bone structure and its anisotropic properties.
To give an idea about the variability of this parameter, Table 2.4 shows the summary of
the ultrasonic wave velocity for different types of human and animals bone tissues.
20
Table 2.4: Summary of the ultrasonic wave velocity with the frequency for different types of human bone tissue and few selected animals (Janson et al 1990).
Studied i
Type of b
Type of i
Frequency [ ]
Speed of d [ / ]
comment
Human
Tibia Frozen 0.1 3526 ± 100 16-30 years 3147 ± 81 58 years 2868 ± 52 55 years
Femur Dry 2 4180 ± 30 Parallel to x axis 3840 ± 30 Angle 45° to x axis 3360 ± 30 Perpendicular to x axis
Tibia
Fresh
5 4300 - 4750 In zones of intersection
diaphysis 1.67 4100 - 4250
0.15 3410 - 3520 3360 - 3780 Lateral direction
Humerus 1
2095 - 2938 Along the bone 2798 - 3408
the middle
diaphysis
Ulna 2366 - 3262 Dog Tibia Frozen 3 - 5 3210 - 3960
Bull
Fresh 10 3620 - 3960
Femur 3 2698 ± 135
Dry 2
3736 - 3782 Parallel to x axis Femur 2589 - 2945 Perpendicular to x axis
Tibia 3335 - 3580 Parallel to x axis 2545 - 2824 Perpendicular to x axis
Phalanx Fresh
5
4030 ± 110 Parallel to x axis 3160 ± 160 Perpendicular to x axis
Dry 4360 ± 170 Parallel to x axis 3270 ± 160 Perpendicular to x axis
Horse Tibia 4.5 3700 —
Pig Femur
Fresh
0.1
2870 - 4541 Healthy bone 1924 - 2250 Bone with breakthrough
i h dh i2442 - 2724 Fragmentary adhesion of b k h h2788 - 3371 Full adhesion of b k h h
Rat Femur
0.15 1960 - 2680
Different density Tibia 2150 - 2500 Humerus 2000 - 2220
2.2.3. Ultrasonic Attenuation
Ultrasonic attenuation is another important parameter which carries a great deal of
information about the physical condition of bone. It depends on beam spreading,
21
scattering, absorption and mode conversion. The predominant attenuation mechanism in
cancellous bone is scattering, while absorption is the predominate attenuation mechanism
in cortical bone. Strictly speaking, the attenuation shows how many times a considered
signal is decreased after propagating within a certain medium and it can be expressed by
a simple equation:
deAA α−= 0 (2.6)
where, α - attenuation, A0 - initial amplitude of the signal, A - amplitude reduced after the
signal traveled through distance d.
In the frequency range clinically used for the ultrasonic bone investigation (0.1 –
2.0 MHz) the ultrasound attenuation is approximately proportional to the frequency and it
can be expressed by the simple formula (Langton and Njeh, 2007):
( ) ff αμ = (2.7)
where ( )fμ - Frequency dependent intensity attenuation coefficient, α – the slope of
attenuation against frequency (dB/MHz/cm) which in clinical practice is known as BUA
– Broadband Ultrasound Attenuation.
In contrast to the velocity, no reliable theoretical relationships between ultrasound
attenuation and the mechanical properties of cancellous bone have been established.
Ultrasonic attenuation is one of the parameters which play a very important role
in the ultrasonic bone quality assessment for i.e. ultrasonic investigation of osteoporosis.
The typical clinical system, which estimates BUA, bases its calculation on the following
equation (Langton and Njeh, 2007):
( ) ( )
( ) ( )bttbb
w TTfAfA
xf lnln868.8
+⎟⎟⎠
⎞⎜⎜⎝
⎛=μ (2.8)
where wA - Maximum peak of the amplitude frequency spectrum calculated from a signal
traveled through water, bA - Maximum peak of the amplitude frequency spectrum
22
calculated from a signal traveled through cancellous bone, tbT and btT - amplitude
transmission coefficient from soft tissue to bone and from bone to soft tissue, assuming
the transmission coefficient between water and soft tissue to be unity.
The slope of attenuation (BUA - expressed in dB/MHz/cm) is a linear fit calculated from
the graph obtained from Eq. 2.8.
2.2.4. QUS - Quantitative Ultrasound
Quantitative ultrasound is a method of an ultrasonic assessment of bone condition
by using various ultrasonic parameters (SOS, BUA, nSOS, nBUA, stiffness). SOS and
BUA were already described in the previous chapters; stiffness is an invented artificial
parameter which helps to properly judge a bone condition based on the measurements of
SOS and BUA. The parameter is called stiffness but the name can be confusing since the
parameter has nothing to do with an actual mechanical stiffness (Young’s modulus).
Ultrasonic stiffness has only a mathematical meaning and it is expressed by the following
formula (Jaworski, 1998):
[ ] [ ] [ ]
2%%% nSOSnBUAstiffness +
= (2.9)
where the two parameters nBUA and nSOS are the normalized parameters of BUA and
SOS. Normalization was made by taking into an account data from a healthy women
population. This means that whenever the new patient is examined, values of BUA and
SOS are being compared with values collected before from a healthy women population.
This is done both for men and women. Mathematical equations used for that operation
come from experimental results and they are as follow (Jaworski, 1998):
[ ] [ ]%10075
50% ⋅
⎥⎦⎤
⎢⎣⎡
−⎥⎦⎤
⎢⎣⎡
=
MHzdB
MHzdBBUA
nBUA (2.10)
23
[ ] [ ]%100180
1380% ⋅
⎥⎦⎤
⎢⎣⎡
−⎥⎦⎤
⎢⎣⎡
=
sm
smSOS
nSOS (2.11)
The examples of experimentally collected values of SOS, BUA and Stiffness obtained
from human and animal populations were collected by the author of the following thesis
and summarized in Table 2.5.
Table 2.5: Review of the ultrasonic parameters of bone basis on a few literature sources.
Investigated parameter Type of tissue Measured values (mean) Source
Ad-SOS* Phalanx 1982 - 2124 [m/s] Rico et al 2001 Ad-SOS* Phalanx 1720 - 2171 [m/s] Bolanowski et al 2005 Ad-SOS* Phalanx 1774 - 1819 [m/s] Gnudi and Ripamoti, 2004 Ad-SOS* Phalanx 1700 - 2100 [m/s] Bolanowski, 2006
SOSL Bovine cortical bone 2850 - 3500 [m/s] Wu and Cubberly, 1997
SOST Bovine cortical bone 1650 - 2000 [m/s] SOS Radius 3900 - 4200 [m/s]
Knapp and Blake, 2001 SOS Tibia 3700 - 4000 [m/s] SOS Phalanx 3700 - 4000 [m/s] SOS Vertebra 3400 - 3800 [m/s] SOS Vertebra 1450 - 1550 [m/s]
Nicholson et al 1998 BUA Vertebra 4 - 15 [dB/MHz/cm] BUA Femur1 ~ 3 [dB/MHz/cm]
Sasso et al 2007 BUA Bovine Femur1 ~ 3 [dB/MHz/cm] BUA Bovine Femur2 5 - 12 [dB/MHz/cm]
Stiffness Calcaneus3 73 - 100 [%] Mautalen et al 1993
Stiffness Calcaneus 4 56 ± 13 [%] Stiffness Calcaneus 3 94 ± 12 [%]
Stevenson et al 1992 Stiffness Calcaneus 4 76 ± 9 [%]
* - Amplitude dependent SOS; 1 – Axis direction; 2 – Direction perpendicular to axis; 3 – Healthy bone (in vivo measurement); 4 – Bone affected by osteoporosis or osteopenia (in vivo measurement); SOSL – Speed of sound (longitudinal wave); SOST – Speed of sound (transversal wave);
24
2.3. References
Black J, Hastings G. 1998. Handbook of Biomaterial Properties. London: Chapman &
Hall, chapters A1 and A2.
Bolanowski M, Jędrzejuk D, Pluskiewicz W, Pruska D. Porównanie badań
ultradźwiękowych z jednoenergetyczną absorpcjometrią rentgenowską w diagnostyce
osteoporozy [Comparison between ultrasonic methods and signle energy X-ray
aborptiometry methods for diagnosis of osteoporosis]. Przegląd Menopauzalny. N1,
pp. 51–55, 2006.
Bolanowski M, Pluskiewicz W, Jawiarczyk A. Przydatność ultradźwiękowego badania
paliczków ręki w ocenie zagrożenia osteoporozy [Usefulness of the ultrasonic
investigation of phalanges for the risk assesment of osteoporosis]. Przegląd
Menopauzalny, N3, pp. 36–42, 2005.
Braidotti P, Brancal FP and Stagnil L. Scanning electron Microscopy of human cortical
bone failure surfaces. J. Biomechanics. V.30, N2, pp. 155 -162, 1997.
Frederick JR. 1965. Ultrasonic Engineering. NY: John Wiley.
Fyhrie DP, Kimura JH. Cancellous bone biomechanics. Journal of Biomechanics, V.32,
pp. 1139-1148, 1999.
Gnudi S and Ripamonti K. Quantitative ultrasound at the phalanxes discriminates
osteoporotic women with vertebral but not with hip fracture. Ultrasound in Medicine
and Bioogy. V.30, N3, pp. 357-61, 2004.
Janson HA, Denis WW, Tatarinow AM. 1990. Ul'trazvukovoye issledovaniye gubchatoy
kosti [Ultrasonic investigation of cancellous bone]. Riga: Zinatne, table 1.3, pp. 25-
26.
Jaworski M. Badania kości ilościową metodą ultradźwiękową [Bone investigation with
quantitative ultrasound methods]. Diagnostyka osteoporozy R S Lorenc, J. Walecki,
Springer PWN, Warszawa, pp. 80-94, 1998.
Kachlik D, Baca V, Cepelik M, Hajek P, Mandys V, Musil V. Clinical anatomy of the
calcaeal tuberosity. Annals of Anatomy V.190, pp. 284-291, 2008.
Knapp KM, Blake GM, Spector TD and Fogelman I. Multisite Quantitative Ultrasound:
Precision, Age- and Menopause-Related Changes, Fracture Discrimination, and T-
25
score Equivalence with Dual-Energy X-ray Absorptiometry. Osteoporosis
International V.12, pp. 456-464, 2001.
Langton CM, Njeh CF. Ultrasonic characterisation of a porous solid, cancellous bone.
Nondestructive Testing and Evaluation, V.14, pp. 257 – 276, 2007.
Lynnerup N, Astrup JG and Sejrsen B. Thickness of the human cranial diploe in relation
to age, sex and general body builds. Head & Face Medicine, V.1, N13, pp. 1-22, 2005
Lynnerup N. Cranial thickness in relation to age, sex and general body build in a Danish
forensic sample. Forensic Science International, V.117, pp. 45-51, 2001.
Mautalen C, Gonzales D and Cirocosta A M. Ultrasonic Assessment of Bone in Normal
and Osteoporotic Women. The Fourth International Symposium on Osteoporosis,
March 27 – 31. 1993, Hong Kong, pp.53.
Nguyen TN and Lethiecq M. Experimental Verification of the Theory of Elastic
Properties Using Scattering Approximations in (0-3) Connectivity Composite
Materials. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,
V.43, N 4, pp. 640-645, 1996.
Nicholson PHF, Muller R, Lowet G, Cheng XG, Hildebrand T, Uegsegger P R, Van Der
Perre G, Dequeker J and Boonen S. Do Quantitative Ultrasound Measurements
Reflect Structure Independently of Density in Human Vertebral Cancellous Bone?
Bone V.23, N5, pp. 425-431, 1998.
Pain HJ. 1985. The Physics of Vibrations and Waves. Chichester: Wiley.
Rho J-Y, Kuhn-Spearing L, Zioupos P. Mechanical properties and the hierarchical
structure of bone, Medical Engineering & Physics, V.20, pp. 92–102, 1998.
Rico H, Aguado F, Arribas I, Hernandez E R, Villa L F, Seco C and Gervas J J. Behavior
of Phalangeal Bone Ultrasound in Normal Women with Relation to Gonadal Status
and Body Mass Index. Osteoporosis International, V.12, N6, pp. 450-5, 2001.
Stevenson JC, Lees B, Ramalingam T, Blake G M. Precision and Sensitivity of a New
Ultrasound Bone Densitometer. The American Society for Bone and Mineral
Research, 14th Annual Meeting, Sept 30 – Oct 4, 1992. Minneapolis, Minnesota,
USA.
26
Teoh SH, Chui CK. Bone material properties and fracture analysis: Needle insertion for
spinal surgery. Journal of the mechanical behavior of biomedical materials, V.1, N2,
pp. 115-39, 2008.
Uklejewski R. 1992. Kość jako wypełniony płynem dwufazowy ośrodek porowaty [Bone
as a liquid filled two-phase porous medium]. Warszwa: IPPT PAN, Wydawnictwo
Spoldzielcze sp. zo.o., chapter 2, pp. 6-10.
Wu J and Cubberly F. Measurement of velocity and attenuation of shear waves in bovine
compact bone using ultrasonic spectroscopy. Ultrasound in Medicine and Biology,
V.23, N1, pp. 129-34, 1997.
27
CHAPTER 3 A Novel Composite Material Specifically
Developed for Ultrasound Bone Phantoms:
Cortical, Trabecular and Skull
3.1. Introduction
In the past years, various groups were involved in the fabrication of ultrasound
bone phantoms using various materials. For instance, Stretizki et al (1997) simply used
the epoxy resin for the cortical matrix to make ultrasound trabecular bone phantoms.
However, the density and acoustic impedance of that material does not agree with the
properties of actual bones. Moreover, the liquid material such as pure epoxy resin
requires more effort and more advanced equipment (such as an oven and a vacuum
chamber) in order to produce phantoms with complex shapes and large scale dimensions.
The problem with eliminating air bubbles also dramatically increases as dimension
increases.
Baykov et al, made a skull bone phantom, using an epoxy resin and tungsten
powder (Baykov et al 2003). The density and velocity of that material worked well (2.15
g/cm3 and 2950 m/s). The value of attenuation coefficient (16.5 dB/cm), since the broad
range of values reported in the literature (shown at the end of this section), is also on the
acceptable level. However, this material is suitable only to mimic a skull bone without
considering the scattering effect from the inner porosity structure. Also, the material
cannot be modified to account for the low and high values of attenuation found in real
skull bones (2-80 dB/cm at 1.5 MHz) (Aubry et al 2003).
Moilanen et al (2004) worked with long bone phantoms (i.e. tibia or femur) made
from PVC. The sound velocity in this material is about ~2300 m/s and it is only enough if
one wants to use it for cortical bone phantoms to investigate the accuracy of measuring
velocity. In the event that the angles related to the refraction coefficient matter and the
28
ultrasonic beam is not perpendicular to the investigated sample, the speed of sound
becomes a very important parameter thus making it necessary to have available a
phantom with acoustical parameters very closely resembling that of an actual cortical
bone.
The following thesis presents a material with designable acoustical properties to
fit the corresponding properties of actual human bones. Moreover, the material can be
used to manufacture various kinds of bone phantoms (cortical, trabecular and skull) in
various shapes and dimensions without using special equipment like a vacuum or
pressure chamber.
3.2. Material: Developing Process and Method of Manufacturing
a) Cortical bone phantoms
Knowing the range values of acoustical parameters in actual bones is essential in
phantom designing. As such, for the ultrasonic wave at frequency 1 MHz, the targeted
speed of sound in cortical bone must be within the range of 3000 - 4000 m/s which is in
agreement with the values reported in the literature (2880 – 3100 m/s (Han et al 1996)
and 3440 – 4220 m/s (Wang et al 1997)). It also fits the discussion presented in
CHAPTER 2. The acoustical attenuation should be around 12 dB/cm at 3 MHz and 19
dB/cm at 5MHz (Lakes et al 1986) which can be extrapolated to ~4 dB/MHz/cm. For the
trabecular bones the speed should be lower (2000 – 3000 m/s) and the attenuation higher
(15 – 30 dB/MHz/cm) (Stretitzki et al 1997).
For the skull bone phantoms, one needs to additionally consider its three layered
structure: two cortical bone layers (inner and outer layers) and a middle trabecular bone
which in this particular case is called diploë (O'Rahilly et al 2004). The acoustical
properties can be considered as a compound of the properties of cortical and trabecular
bones. The overall values of sound velocity and attenuation are also expected to be
between the corresponding values for cortical and trabecular bones.
According to data from other literature, the value of attenuation in the actual skull
is very broad. The difference in the results can be attributed to many factors, including
29
different porosity, different physical skull bone condition, and even different
measurement techniques. For example, Aubry et al (2003) reported sound absorption by
skull bone in the range 2-80 dB/cm at 1.5 MHz within the same ex-vivo skull. Pichardo et
al (2011) reported 27 dB/cm attenuation for cortical bone at 1.4 MHz. Attenuation range
from 33 dB/cm to 105 dB/cm was measured at 2 MHz by Ammi et al (2008). Barger and
Fry (1978) give 26 (dB/cm) for cortical bone and 52, 121 (dB/cm) for two different
diploe skulls at 2 MHz. Pinton et al (2012) reported 16.6 dB/cm at 1 MHz, which can be
extrapolated to ~33 dB/cm at 2 MHz. Baykov et al (2003) reported the range between 14-
19 dB/cm at 1.7 MHz from the ex-vivo measurements.
Similar to the attenuation, the speed of sound in the skull bone is also broad and is
within the range 2000 – 4000 m/s due to the density and inner porosity of the structure
(Connor et al 2002). The material presented in this thesis is in agreement with this broad
range. It is also important to point out that the procedure allows forming skull bone
phantoms with any properties as desired. Moreover, one can notice the relationship that
higher porosity and lower density decreases the observed sound velocity and increases
the attenuation. This phenomenon can also be reproduced with the phantoms described in
this thesis.
All of these features can reproduced only if one finds a material which is
acoustically very close to actual bones. Moreover, the material should be easily
modifiable in order to create bone phantoms with various density, sound velocity and
attenuation. To approach such a goal, it was necessary for the author to do extended
research on all available materials in the modern market. The task was not easy, since the
manufacturers usually do not provide any information about the acoustical properties of
the materials which they are selling. What one can find are the simple properties like
density, melting point and only sometimes the mechanical properties like i.e. Young's
modulus. Even mechanical properties which are slightly less common like bulk and shear
modulus are already much harder to find. The speed of sound and attenuation are usually
never provided and the only way to check them is to find a proper publication done by
some other research group or do the tests empirically.
30
The author, after a long research and experimental tests, focused mainly on
polymer based composite materials. This is due to a variety of polymers available in the
modern market with variety of properties (including acoustical properties).Some of the
polymers even without any modification are already acoustically fairly close to the bones.
The problem was to find a material that can be easily manufactured without any
expensive industrial equipment (i.e. vacuum or pressure chamber). Of course, it has to
also maintain the properties of bones. Moreover, the material should be modifiable to
allow changes in its density, sound speed and ultrasonic attenuation.
As a solution, the author of the following thesis proposes using an epoxy resin as
a base material and adding a certain type of micro-particles in order to improve and
modify its properties. A pure epoxy resin has a speed of sound on a level about 2500 m/s
(Table 3.1) and this value is already fairly close to the targeted 3000 m/s. However, a
pure epoxy resin does not have a capability of modifying its properties and there is no
way of increasing its speed of sound without adding an extra component. This forced the
author to look for an extra ingredient which could significantly change the properties of
the epoxy resin. In this case, the author focused on a technology of composite materials.
The preliminary theoretical work (mostly a literature review) shows a potential in such a
technology and it encouraged the author to move to the next step which is experimental
work. As a result of the research, the second ingredient was introduced to the epoxy
resin. This ingredient is an alumina powder with properties summarized in Table 3.1.
Table 3.1: Summarized properties of the ingredients used for the new material.
Material Sound Velocity [m/s]
Attenuation at 2.25 MHz [dB/cm]
Density [g/cm3]
Acoustic Impedance [MRayl]
Source
Epoxy resin 2500 ± 20 7.5 ± 0.3 1.11 ± 0.01 2.8 ± 0.1 experiment Alumina powder
11000 - * 3.95 43.45 Munro, 1997
* Ultrasound attenuation of alumina powder cannot be estimated due to small size of particles
As it was pointed out in CHAPTER 2, the added particles have to be significantly
smaller than the wavelength of ultrasound. In this case, the author chose alumina particles
which are comparable with the diameter of osteon (10 - 500 µm). The SEM picture of
31
alumina powder which shows the desired particles' size is shown in Figure 3.1. In order
to make a bone phantom, the two components (epoxy and alumina) can be mixed
together with various ratios to get various densities and certain acoustical properties. One
of the big advantages of this technology is its simplicity: the material can be hand mixed
without any industrial equipment. The only necessary tool is just a kitchen scale.
Figure 3.1: SEM picture of alumina powder is showing the targeted diameter of particles.
The developed material on the stage of its preparation (before it is formed to the
desired shape) is shown in Figure 3.2. It should be noted that its unique properties i.e.
viscosity, curing time and curing temperature allows the manufacturer to create any bone
phantom with all kinds of shapes and with all desired acoustical properties which
characterize human bones (speed of sound, attenuation and acoustic impedance). All that
is necessary, is to have a proper mold, which should be a negative of an actual desired
phantom shape. The material should be carefully positioned into the mold, after which,
the only thing which one has to be aware of, is the curing time, which depends on the
type of used epoxy resin and a temperature of surrounding environment. In the case of the
materials used by the author, after mixing, stirring and forming process, the created
composite has to be stored at room temperature for at least 16 hours and then optionally
32
moved into the oven for 1 hour at 93°C. The temperature treatment does not change its
acoustical properties but it changes its thermal properties which is convenient if one
wants to use phantom in high temperature conditions.
Figure 3.2: The material for bone phantom during the preparation process.
b) Trabecular bone phantoms
The trabecular bone phantoms with proper ultrasonic properties can be made out
of the porous version of the developed material for cortical bone phantoms. This can be
done by mixing the previously described material with small particles (diameter ~1mm),
which maintain similar acoustical properties to actual bone marrow. Following that idea,
four different phantoms were manufactured from the previously described bulk material,
with one chosen density (composition). The material was mixed with small (1.0 ± 0.5
mm) granules in order to obtain a simple version of a porous structure. Samples prepared
in this way do not have an open porosity structure as found in actual trabecular bones but
it is sufficient for the purpose of this thesis to present one possible type of phantom with
relatively simple structure, which has satisfactory ultrasonic properties. This was
confirmed by the experimental results presented in CHAPTER3.3.
The granules can be produced from a variety of materials that have similar
acoustical properties to bone marrows. They can be found in two forms; artificially
manufactured or naturally existing; i.e. poppy seeds. The samples prepared for
experimental purposes have four different porosity levels and are shown in Figure 3.4.
33
The porosity level of the prepared trabecular bone phantoms were estimated using
a specially written MATLAB program (Figure 3.3). The program can calculate the ratio
between surface areas of pores (black regions) and the matrix (white material) from the
optical image of porous samples. The calculated porosity and the optical images of each
sample used for the calculations are shown in Figure 3.4.
Figure 3.3: Program window for calculation of surface porosity of porous samples. Left window - raw image of sample; right window - image converted to black and white colors; center panel -
options and calculated porosity
The author is aware that surface porosity calculated in this way does not need to
reflect the total volume porosity. However, if one assumes the homogeneity of the
phantoms, then the porosity within the total volume is similar to the porosity on the
surface.
Figure 3.4: The prepared porous phantoms with calculated porosity level: 14%, 19%, 22% and 30% (trabecular bone phantoms).
34
3.3. Methods of Investigation of a Developed Material
For experimental purposes, one reference sample made out of a pure epoxy resin
and twenty one distinct samples with different compositions (literally different ratios
between alumina powder and epoxy resin) were prepared and investigated in order to
estimate the relationship between the ultrasonic properties and the density of the material.
The prepared cortical bone phantoms are shown in Figure 3.5.
The level of the homogeneity and uniformity of the samples were estimated by
measuring the sound velocity in three different perpendicular directions for each sample,
as shown in Figure 3.6.
Figure 3.5: The 21 prepared for the experimental purposes bone phantoms with different composition and the epoxy sample as a reference.
The experiments consist of investigation of two main measuring parameters:
speed of sound and ultrasound attenuation. Both parameters were estimated in relation to
the density of the developed material. The experiments were performed according to the
following procedure: two ultrasonic transducers (transmitter and receiver) were placed
coaxially on both sides of the sample. The signal was transmitted from the ultrasound
wave generator 33210A (Agilent, USA) and recorded using digital oscilloscope TDS
2024B (Tektronix, Beaverton, OR, United States).
35
The speed of sound and ultrasonic attenuation was measured using two sets of
two transducers (Technisonic, USA) at two frequencies for each set: 1MHz and
2.25MHz. These frequencies are in the range commonly used for bone analysis in many
medical applications (including osteoporosis investigation and ultrasonic brain imaging
through the human skull which is the subject of the research in our group (Sadler et al
2011)).
Figure 3.6: Directions of samples and directions of the propagation of ultrasound waves.
The speed of sound analysis was based on the transmission method. This literally
means that the two transducers were placed coaxially on the opposite sides of the sample.
The signal was transmitted from the transmitter and recorded by the receiver after it
passed through the investigated sample. Knowing the thickness of the sample and
assuming a normal incidence wave, it is possible to calculate the speed of a longitudinal
wave which propagates through the medium using Eq. 2.1.
The speed of sound was measured in three different perpendicular directions
(Figure 3.6) and the experiments showed that the samples are fairly isotropic (difference
between velocities in each direction is about ~1% which is within statistical error). For
this reason the sound velocity values measured along all three directions were averaged
and the average was used for further analysis.
The acoustical attenuation analysis was also based on the transmission mode.
Firstly, the two transducers were placed coaxially, facing each other, in a water tank at
36
20° C (Figure 3.7). The ultrasonic wave propagated in water was recorded and was
considered as the reference signal. Secondly, the samples were inserted into the water
between the transducers. The ultrasonic wave signal for each prepared sample was
recorded in the same way as the referenced signal immersed in water. The value of the
attenuation was calculated as the average of the values measured at 10 different points for
each of the samples.
The ultrasonic attenuation was calculated from the following formula (Sasso et al
2008):
( )( ) ( )⎟⎟
⎠
⎞⎜⎜⎝
⎛−⋅= 21log20 R
fAfA
d p
wα (3.1)
where Aw is the amplitude of the wave transmitted in water before the sample was
inserted, Ap is the amplitude of the transmitted wave in water after the sample was
inserted between transducers. R is the reflection coefficient:
12
12
ZZZZR
+−
= (3.2)
Z2 is the acoustic impedance of phantom and Z1 is the acoustic impedance of water.
The densities of the samples were measured using the densitometer YDK 01
(Sartorius AG, Goettingen, Germany) by using formula (Sartorius manual 2001):
( )
3
3
0012.099983.0
0012.0
cmg
Gcm
gaw w
+⋅
⎥⎦⎤
⎢⎣⎡ −⋅
=ρ
ρ (3.3)
where w(a) is the sample weight in the air, ρw is the density of water at 20° C (0.99823
g/cm3).
( ) ( )flwawG −= (3.4)
where w(fl) is the sample weight in the water.
37
Figure 3.7: Experimental set-up used for the investigation of attenuation of the prepared phantoms.
The samples with small dimensions required small transducers and a narrow
ultrasonic beam to perform all the experiments. Two pairs of flat transducers were
chosen: 2.25 MHz with 0.25” diameter and 1 MHz 0.75” diameter. The larger diameter of
the second transducer required using a small hole which decreased the size of the
ultrasonic beam to the desired dimensions (small enough to entirely propagate through
the sample). The pinhole was made with a specially designed highly attenuative rubber
material which blocked the entire unwanted signal from the edges (Figure 3.7).
The frequency bands of the transducers used in all experiments are shown on the
recorded spectra in water which are presented in Figure 3.8 and Figure 3.9. The graphs
show that 1 MHz transducer emits a wave which consists of frequency components
within the range 0.75 to 1.15 MHz. The ultrasonic wave emitted by transducer at 2.25
MHz consists of components with the frequency range 1.9 to 2.6 MHz. This shows that
all the measured parameters such as sound velocity and attenuation are actually measured
not for the single frequency peak but for the entire range of spectrum. This must be kept
in mind if one were to compare the obtained results with results found in other literature.
38
Figure 3.8: Spectrum of the recorded signal for 1 MHz transducer in water without any sample.
Figure 3.9: Spectrum of the recorded signal for 2.25 MHz transducer in water without any sample.
For the statistical error analysis, in order to judge a certainty of any measured
parameters (i.e. sound speed, attenuation) the author calculated Standard Deviation and
the 99% Confidence Intervals (with t critical value 2.977) for all investigated samples
from the following formulas:
( )21
1 xxn
s i
n
i−= Σ
= (3.5)
( )nsvaluecriticaltx ± (3.6)
where s is the standard deviation, n is the number of samples, x is the mean value.
39
3.4. Properties of the Material - Results
3.4.1. Flat Non-porous Bone Phantoms
a) The speed of sound and elastic modulus analysis of samples with different
densities
The samples prepared from the material without any porosity layer (cortical bone
phantoms - Figure 3.5) were investigated in order to estimate the relationship between the
speed of sound and the density. The obtained results from all samples are graphically
presented in Figure 3.10. The most important density range which corresponds to the
interested region of the human cortical bones (2.00 – 2.22 g/cm3) was additionally
magnified. This range is close to values 2.068 ± 0.142 g/cm3 reported by Numbenjapon et
al (2007) for a healthy control group.
The graph in the Figure 3.10 shows that sound velocity depends strongly (almost
exponentially) on the density of the sample. Moreover, it was also clearly defined that
there is no significant correlation between the velocity and the investigated frequency,
which is in agreement with the data reported in literature (Wu and Cubberley 1997).
The obtained values are close to the values of longitudinal velocity for an actual
cortical bone found in the literature (for example Stretitzki et al 1997, Wu and Cubberley
1997, Lang 1970). For instance, depending on the direction and density, the speed of
sound varies from 3300 m/s to 4300 m/s for human bones (Stretitzki et al 1997) and from
3000 to 3300 m/s (Wu and Cubberley 1997) and even up to 4000 m/s for bovine cortical
bones (Lang 1970) which are used to mimic human bones in many applications.
40
Figure 3.10: The relationship between velocity and density for the developed material for two frequencies: 1 MHz and 2.25 MHz for the density range 1.10 – 2.22 g/cm3.
The elastic modulus of the developed material was also analyzed based on results
obtained from the ultrasonic measurements and Eq. 2.5. It was noticed that the
relationship between the bulk modulus and the density is close to exponential trend which
means that the bulk modulus is growing much faster than the density. The data analysis is
graphically presented in Figure 3.11.
41
Figure 3.11: The relationship between the logarithm of bulk modulus and logarithm of density for the developed composite material for the ultrasonic bone phantoms.
b) Attenuation analysis of samples with different densities
The ultrasound attenuation was measured for the same set of 21 samples
described previously and using the same transducers at 1MHz and 2.25 MHz. The
obtained results of the full density range and the magnification of the most interested
region (density range of actual cortical bones) are shown in Figure 3.12.
The results of the experiments that we obtained, unfortunately, did not
demonstrate a clear trend in the attenuation versus density graph. However, according to
the well known prediction, the attenuation is higher for the higher frequency of the
ultrasonic wave. Moreover, the measured values are in agreement with the values of
attenuation observed in bovine cortical bones (Sasso et al 2008).
y = 1.739x + 4.4895R2 = 0.9833
9.6
9.7
9.8
9.9
10
10.1
10.2
10.3
10.4
10.5
2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4 3.45
Log(density)
Log(
bulk
mod
ulus
)
42
Figure 3.12: The relationship between ultrasound attenuation and density for the developed material for the density range 1.10 – 2.22 g/cm3.
3.4.2. Various Phantoms and their Properties
a) Trabecular bone phantoms – porous samples
The previously described trabecular bone phantoms (Figure 3.4) were investigated
in order to measure sound velocity, attenuation coefficient, density and porosity. All the
obtained parameters of these phantoms are summarized in Table 3.2. One can see that the
porosity is linearly correlated with the density due to low pore (granules) density which
decreases the total density of the prepared phantoms. The calculated linear coefficient is
R2 = 0.99.
The investigation of the ultrasonic attenuation and the speed of sound showed an
existing dependence on the porosity and frequency. For instance, the relationship
between the velocity and porosity is shown in Figure 3.13. The downward trend on the
graph is in agreement with the trend found in literature (Lee 2011). The plotted data also
43
show that the speed of sound is higher for the higher frequency which also correlates to
the results previously reported in literature (Hakulinen et al 2006). Moreover, the
measured attenuations (18.9 – 24.9 dB/cm at 1 MHz and 33.5 – 39.4 dB/cm at 2.25 MHz)
are close enough to the values found in the literature which are between 10 – 25 dB/cm
for the frequency 1 – 2.25 MHz (Wear 2001) and ~25 dB at 1 MHz and ~40 dB at 2.25
MHz (Hakulinen et al 2006).
Table 3.2: The properties for the trabecular bone phantoms.
Sample Density [g/cm3]
Porosity [%]
Speed of sound [m/s]
99% Confidence
Intervals [m/s]
α [dB/cm]
99% Confidence
Intervals [dB/cm]
A 1 MHz 1.90 ~14 2862 ± 10 2845 - 2879 18.9 ± 0.7 18.2 – 19.5 2.25 MHz 2923 ± 10 2906 – 2940 33.5 ± 1.0 32.5 – 34.4
B 1 MHz 1.85 ~19 2828 ± 10 2810 - 2845 21.8 ± 1.0 20.8 – 22.7 2.25 MHz 2885 ± 10 2868 - 2902 39.0 ± 0.7 38.3 – 39.6
C 1 MHz 1.83 ~22 2817 ± 10 2800 - 2835 22.2 ± 0.4 21.8 – 22.6 2.25 MHz 2832 ± 10 2815 - 2849 39.3 ± 0.7 38.6 – 40.0
D 1 MHz 1.76 ~30 2720 ± 10 2703 - 2737 24.9 ± 0.5 24.4 – 25.4 2.25 MHz 2760 ± 10 2743 - 2777 39.4 ± 0.3 39.1 – 39.7
Figure 3.13: Velocity dependence on porosity for the prepared trabecular bone phantoms.
2650
2700
2750
2800
2850
2900
2950
10 15 20 25 30 35
Porosity [%]
Vel
ocity
[m/s
]
1 MHz
2.25 MHz
44
b) Skull bone phantoms
The developed material can be used for bone phantoms, and manufactured for
many other purposes. Some examples of the simplest usages are the flat models of skull
bone (Figure 3.14) and phantoms with one flat and one undulated surface (Figure 3.15).
Both sets of phantoms were made with and without an inner porosity layer.
The acoustic properties of these phantoms were investigated with an ultrasonic
flat transducer at a nominal frequency of 2.25 MHz in a way similar to previous
experiments. The properties are summarized in Table 3.3. Additionally, the broadband
ultrasound attenuation was calculated for both samples. The method used for calculations
is based on the comparison of two frequency spectra of two ultrasonic signals: wave
transmitted in water without sample and wave transmitted in water after the sample was
inserted between two transducers. Amplitude of each frequency peak from the first
spectrum was divided by the corresponding frequency amplitude from the second
spectrum and the attenuation was calculated based on the obtained ratio. All calculations
based on the described algorithm were done with using a specifically prepared program
in MATLAB which user interface is shown in Figure 3.16. The program needs as an
input two A-Scans: the reference signal for water and the signal in water after the sample
was inserted between two transducers. It is also necessary to provide the thickness of
sample and its reflection coefficient. Based on the provided inputs the program calculates
the attenuation from the signals in time domain and the broadband ultrasound attenuation
from the frequency spectra. The results are presented in graph form in Figure 3.17.
The simple shape of a flat phantom is an advantage as it allows for an exact
measurement of the acoustic properties unlike in real bones due to its complex
geometrical structure. One should be also aware that the flat samples and flat-undulated
samples can be considered purely for laboratory research purposes.
45
Figure 3.14: Flat phantoms made from the composite material with and without porosity layer.
Figure 3.15: Flat-wrinkled phantoms without (upper) and with (lower) porosity layer.
46
Table 3.3: Ultrasonic properties of the flat and flat-wrinkled phantoms with and without porosity layer (diploe).
Property Phantom4A Phantom 4B Flat-wrinkled Flat-wrinkled Density [g/cm3] 2.09 2.04 ~2.1* ~2.0*
Acoustic Impedance [MRayl]
6.3 6.1 ~6.3* ~6.0*
Sound velocity [m/s] 3030 ± 20 2971 ± 20 3011 ± 30 2989 ± 10 99% Confidence Intervals
[m/s] 3011 - 3949 2952 - 2990 2983 - 3039 2979 - 2998
Attenuation @ 2.25MHz [dB/cm]
7.6 ± 0.4 18.8 ± 0.7 9.7 ± 0.3 15.7 ± 0.7
99% Confidence Intervals [dB/cm]
7.2 – 8.0 18.1 – 19.4 9.4 – 10.0 15.0 – 16.3
BUA (central frequency 2.25MHz) [dB/MHz/cm]
3.1 ± 0.3 5.2 ± 1.0 2.9 ± 0.1 3.9 ± 1.5
99% Confidence Intervals [dB/MHz/cm]
2.8 – 3.4 4.2 – 6.1 2.8 – 3.0 2.5 – 5.3
* - Due to big dimensions of the phantoms, the values were estimated based on the measurements made on a small piece of phantoms.
Figure 3.16: User interface of the program written in MATLAB for the calculations of ultrasonic attenuation.
47
Figure 3.17: Broadband ultrasound attenuation for flat samples with and without diploe layer (measurements were made by using ultrasonic transducer at nominal frequency 2.25MHz).
In order to achieve results as close to those from a biological tissue, a more
complex and advanced research study is inevitable. Designing and manufacturing
phantoms with greater results using various pathology conditions is a necessity to
achieving our goal of mimicking natural bones.
The developed material allows a manufacturer to create bone phantoms in a
variety of shapes and dimensions. Even the most complex shape (related to the skull), the
calvaria part of the human skull, including all the wrinkles on its surface, can be
successfully manufactured using the developed material which our group has done
successfully shown in Figure 3.18.
48
Figure 3.18: The skull bone phantoms with real profile with and without middle porosity layer (diploe).
3.5. Discussion
The newly developed materials for ultrasonic bone phantoms were investigated at
two frequencies: 1 MHz and 2.25 MHz. However, the emitted ultrasonic waves are not
monochromatic and they consist of frequency broadband which is visible on the
calculated spectra presented in Figure 3.8 and Figure 3.9. This should be noticed if one
wants to compare the results from our study with the values of actual bones reported in
other literature.
49
The results from all experiments demonstrated that the values of sound velocity
and attenuation are very close to the values observed in other literatures pertaining to
actual cortical and trabecular human bones. However, one should be aware that all
measured values depend on the frequency of ultrasonic wave due to dispersive properties
of the developed material. For instance the sound velocity measured at higher frequency
could also be higher and this incident can be investigated in the future. For certain
applications (i.e. acoustic microscopy), the material can be slightly modified in order to
achieve the appropriate values of acoustical properties. However, the investigation done
for the frequency range: 1 - 3 MHz (Figure 3.8 and Figure 3.9) shows no significant
dispersion relation in the cortical bone phantoms. Dispersion was only observed in the
trabecular bone phantoms, which is in agreement with the behavior of actual trabecular
bones. Moreover, the ultrasound frequencies used in our experiments are in the range of
frequencies commonly used in modern medical devices which deal with bone structures
in the human body (i.e. osteoporosis investigation and brain imaging through the human
skull). In this case the developed material, according to the authors’ knowledge, can meet
all the requirements which are necessary for manufacturing ultrasound bone phantoms in
various applications.
3.6. Conclusions
The developed material successfully mimics required acoustical properties of the
human cortical bones (sound velocity, attenuation, density and acoustic impedance). The
ability to add an inner porosity layer is a key advantage which allows us to achieve our
goal - manufacturing trabecular bone phantoms with satisfactory acoustical properties.
Depending on the nature of the granules used as pores, the acoustical properties can be
modified in order to achieve the exact required properties of the human trabecular bones
(healthy or affected by osteoporosis). However, the developed phantoms do not mimic
the anatomical and microscopic structure of real bones, but such goal was not intended in
current research. In fact, they mimic the acoustical properties (at the frequencies 1 MHz
and 2.25 MHz, investigated in our laboratory) which was the target of the research. In
this particular case, the developed material is suitable for manufacturing phantoms for
various biomedical and laboratory applications. Such phantoms can be successfully used
50
for specific research purposes affiliated with developing ultrasonic diagnostic and
therapeutic methods, and for educational purposes, but first and foremost for our
examination of osteoporosis or/and brain structure through the human skull bone (Wydra
et al 2013, Sadler et al 2011, Shapoori et al 2010) which is an area of high research
priority within our group.
3.7. References
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Aubry JF, Tanter M, Pernot M, Thomas JL, and Fink M. Experimental demonstration of
noninvasive transskull adaptive focusing based on prior computed tomography scans.
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Baykov SV, Babin LV, Molotilov AM, NeÏman SI, Riman VV, Svet VD, and Selyanin
AI. Physical and Technical Aspects of Ultrasonic Brain Imaging through Thick Skull
Bones: 2. Experimental Studies. Acoustical Physics. V.49, N4, pp. 389–395, 2003.
Translated from AkusticheskiZhurnal, V49, N4, pp. 465–473, 2003.
Connor CW, Clement GT and Hynynen K. A unified model for the speed of sound in
cranial bone on genetic algorithm optimization. Physics in Medicine and Biology.
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Fry FJ, Barger JE. Acoustical properties of the human skull. J. Acoust. Soc. Am. V.63,
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Hakulinen MA, Day JS, Toyras J, Weinans H and Jurvelin JS. Ultrasonic characterization
of human trabecular bone microstructure. Physics in Medicine and Biology. V.51 pp.
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Han S, Rho J, Medige J and Ziv I. Ultrasound Velocity and Broadband Attenuation over
a Wide Range of Bone Mineral Density. Osteoporosis International. V.6, pp. 291-
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51
Lakes R, Yoon HS and Katz JL. Ultrasonic wave propagation and attenuation in wet
bone. J. Biomed Eng. V.8, N2, pp. 143-8, 1986.
Lang SB. Ultrasonic Method for Measuring Elastic Coefficients of Bone and Results on
Fresh and Dried Bovine Bones. IEEE Transactions on Bio-Medical Engineering,
V.BME-17, N2, pp. 101-105, 1970.
Lee KI. Correlations of group velocity phase velocity and dispersion with bone density in
bovine trabecular bone. J. Acoust. Soc. Am. V.130, N6, pp. 399-404, 2011.
Moilanen P, Kilappa V, Nicholson PHF, Timonen J, and Cheng S. Thickness Sensitivity
of Ultrasound Velocity in Long Bone Phantoms. Ultrasound in Med. & Biol., V.30,
N11, pp. 1517–1521, 2004.
Munro RG. Evaluated Material Properties for a Sintered alpha-Al2O3. Journal of the
American Ceramic Society, V.80, pp. 1919-1928, 1997.
Numbenjapon N, Costin G and Pitukcheewanont P. Low Cortical Bone Density
Measured by Computed Tomography in Children and Adolescents with Untreated
Hyperthyroidism. Journal of Pediatrics. V.150, pp. 527-30, 2007.
O'Rahilly R, Müller F, Carpenter S, Rand R. 2004 Basic Human Anatomy: A Regional
Study of Human Structure (Dartmouth Medical School, e-book).
Pichardo S, Sin VW and Hynynen K. Multi-frequency characterization of the speed of
sound and attenuation coefficient for longitudinal transmission of freshly excised
human skulls. Physics in Medicine and Biology. V.56, pp. 219–250, 2011.
Pinton G, Aubry JF, Bossy E, Muller M, Pernot M, and Tanter M. Attenuation,
scattering, and absorption of ultrasound in the skull bone. Med. Phys. V.39, N1, pp.
299-307, 2012.
Sadler J, Shapoori K, Malyarenko E, Di Carlo A, Dech J, Severin F and Maev R.Gr.
Resolving the Location of Acoustic Point Sources Scattered Due to the Presence of a
Skull Phantom. Acoustical Imaging. V.30, N5, pp. 271-278, 2011.
52
Sartorius YDK 01, YDK 01-0D, YDK 01LP Density Determination Kit User’s Manual.
June 2001, Sartorius AG, Goettingen, Germany.
Sasso M, Haiat G, Yamato Y, Naili S, Matsukawa M. Dependence of ultrasonic
attenuation on bone mass and microstructure in bovine cortical bone. Journal of
Biomechanics. V.41, pp. 347–355, 2008.
Shapoori K, Sadler J, Malyarenko E, Severin F; Boni E,Ramalli, A, Tortoli P, Maev RG.
Adaptive beamforming for ultrasonic phased array focusing through layered
structures. Ultrasonics Symposium (IUS), 2010 IEEE, pp. 1821 – 1824.
Stretitzki R, Evans JA and Clarke AJ, The Influence of Porosity and Pore Size on the
Ultrasonic Properties of Bone Investigated Using a Phantom Material. Osteoporosis
International. V.7, pp. 370-375, 1997.
Wang SF, Chang CY, Shih C and Teng MMH, Evaluation of tibial cortical bone by
ultrasound velocity in oriental females. The British Journal of Radiology. V.70 pp.
1126-1130, 1997.
Wear KA, Ultrasonic Attenuation in Human Calcaneus from 0.2 to 1.7 MHz. IEEE
Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. V.48, N2, pp.
602-608, 2001.
Wu J and Cubberley F. Measurement of velocity and attenuation of shear waves in
bovine compact bone using ultrasonic spectroscopy. Ultrasound in Med. & Biol.,
V.23, N1, pp. 129 – 134, 1997.
Wydra A, Malyarenko E, Shapoori K, Maev R Gr. Development of a practical ultrasonic
approach for simultaneous measurement of the thickness and the sound speed in
human skull bones: a laboratory phantom study. Physics in Medicine and Biology.
V.58, pp. 1083-1102, 2013.
53
CHAPTER 4 A Theoretical Prediction of Acoustical
Properties of the Developed Composite Material
The results showed in CHAPTER 3 come from the experimental work. This
chapter introduces the physics behind the developed composite material and helps to
understand the origin of measured properties. In this case, the author introduces four
distinct mathematical models which are commonly used to evaluate and calculate the
properties of composite materials.
All models assume the case when the ultrasonic wavelengths are much larger than
the particle size. In practice, this means that the models can be theoretically valid for the
frequency range lower than ~10 MHz. The simplest models are those from Reuss (1929)
and Voigt (1910). The first one assumes constant stress throughout the solid; the second
one assumes constant strain. Both lead to extreme upper and lower limits which were
noted by Nguyen and Lethiecq (1996). Devaney and Levine (1980) have proposed a
model which is based on a self-consistent formulation of multiple-scattering theory.
Twelve years later, Berryman (1992) proposed more complicated models based on
single-scattering approximations: the average T-matrix approximation (ATA); and the
coherent potential approximation (CPA). It is necessary to point out that all approaches
assume that the inclusions are spherical with same diameter.
4.1. Reuss’ and Voigt’s Model
The simplest theoretical models used for approximations of mechanical properties
of composite materials have already been known for about 100 years and they come from
Reuss (1929) and Voigt (1910). The Reuss’ model assumes that the total strain is a sum
of strains on individual particles. On the other side, the Voigt’s model assumes similar
conditions but for the stress: the total stress is a sum of stress from all particles. The
Voigt’s model gives a following formula for calculating a Young’s modulus of composite
material:
54
( ) ppmpcomp EVEVE +−= 1 (4.1)
where pV - particle volume fraction, mE - matrix Young’s modulus, pE - particles
Young’s modulus.
However, since the particles’ volume fraction for the developed composite is
relatively low and the reinforcement exist discontinuously; the Voigt’s model does not fit
to the considered case. Therefore, the prediction of elastic modulus should follow the
Reuss’ model which assumes the conditions closer to our case. For the iso-stress
condition the equation for Young’s modulus looks as follow:
p
p
m
p
comp EV
EV
E+
−=
11 (4.2)
Knowing the Young’s modulus and several basic mechanical equations (Chawla, 1998):
( ) ( )vGvK
GKKGE +=−=+
= 122133
9 (4.3)
( )( ) ( )v
Ev
vKEK
KEG+
=+−
=−
=1212
2139
3 (4.4)
( )( )
( ) ( )vE
vvG
EGEGK
21321312
33 −=
−+
=−
= (4.5)
where v is a Poisson’s ratio, K and G are bulk and shear modulus of composite; One can
calculate the longitudinal sound velocity using Eq 2.4 and data from Table 4.1; the result
is shown in Figure 4.1.
Table 4.1: Mechanical properties of components used for the developed composite material for ultrasound bone phantoms.
Material Bulk modulus (K) [GPa] Shear modulus (G) [GPa] Source Epoxy 4.7 1.7 experiment
Alumina powder 228 152 Accuratus, 2013
55
4.2. Average T-Matrix Approximation (ATA)
The model was created in terms of scattering phenomena for long wavelengths
and it is a single-scattering approximation which neglects the multiple-scattering
contributions from all particles. The ATA model was cited by Nguyen and Lethiecq
(1996) and it is given by the following equations:
12
2
11
1
1 34
34
34
1
GK
V
GK
V
GK ++
+=
+ (4.6)
12
2
11
1
1
1FG
VFG
VFG +
++
=+
(4.7)
11
1111 2
896 GK
GKGF++
×= (4.8)
where indices 1 and 2corresponds to matrix and inclusions respectively.
The ATA model was tested by the author of the following thesis in order to
estimate and predict the sound velocity of the developed composite material. The
mechanical properties were calculated from Eq. 4.6, 4.7 and 4.8 (using data from Table
4.1) and the sound velocity was calculated from Eq. 2.4. The results are presented in
Figure 4.1.
4.3. Coherent Potential Approximation (CPA)
CPA model assumes that the single-scattering contributions from all inclusions
are equal to zero and the multiple-scattering effects are not taken into an account. The
model is represented by the following equations (Nguyen and Lethiecq, 1996):
GK
V
GK
V
GK34
34
34
1
2
2
1
1
++
+=
+ (4.9)
56
FGV
FGV
FG ++
+=
+ 2
2
1
11 (4.10)
GKGKGF
289
6 ++
×= (4.11)
Similarly like with the previous models, the sound velocity was calculated from Eq. 2.4
(using Table 4.1) and the obtained values were placed in the graph presented in Figure
4.1.
4.4. Devaney Model
The model considers a multiple-scattering theory and it is based on a self-
consistent formulation (Devaney and Levin, 1980). It is interesting to note that for the
case when the wavelengths are much larger than the inclusions and their concentration is
low, this model behaves very similar to CPA. When the multiple-scattering cannot be
neglected (for instance when the concentration of inclusions is important) the Devaney
model differs markedly from the CPA model. The mechanical properties of composite
material given by the Devaney model are coming from the following equations cited by
Nguyen and Lethiecq (1996):
( )( )( )12
1221 343
43KKGK
KKGKvKK−++−+
+= (4.12)
( ) ( )( ) ( )( )1262015
435
2
1221 GGGKGGK
GGGGKvGG−+++
−++= (4.13)
In order to check the validity of the Devaney model for the developed composite
material for ultrasound bone phantoms, the mechanical properties were calculated from
Eq. 4.12 and 4.13 (using Table 4.1), and the sound velocity was calculated using Eq. 2.4.
The calculated values are shown in Figure 4.1.
57
4.5. Summary
The longitudinal sound velocity of the developed composite material was already
measured experimentally for distinct densities and such research was described in
CHAPTER 3. This chapter (CHAPTER 4) introduces the sound velocity calculated from
four theoretical models (Reuss’ model, ATA, CPA and Devaney model) and compares
calculated values with the actual experimental data. All results from the experimental and
theoretical work are summarized in graph and shown in Figure 4.1. One can notice that
the two first models (Reuss’ model and ATA) do not provide satisfactory results and they
are significantly different from the obtained experimental values. This means that those
models do not consider all required conditions and they cannot be used for any
approximation purposes to predict the ultrasonic behaviour of the described above
composite material.
The two last models (CPA and Devaney) provide much better results, and the
calculated sound velocities are very close to those obtained from the performed
experiments. It is also important to note that those two models almost overlap with each
other for the range of low particle volume ratio (less than ~30%) and they start to split
into two different trends for the ratio above ~30%. This is in agreement with the
theoretical assumptions made for the models, which means that they should behave fairly
the same for the range where multiple-scattering effects are negligible (Nguyen and
Lethiecq, 1996).
58
Figure 4.1: Comparison between calculated from the theoretical models and experimentally measured longitudinal sound velocity.
The made assumptions for the models (CPA/Devaney) can also explain small
differences between and the experimental results and the calculated values. Both models
assumed that the composite consists only of the spherical inclusions with constant radius.
This is obviously not true and in a real case the particles look more like random-shaped
solids, discs or even flakes. The differences may appear also due to the taken values of
mechanical properties of epoxy resin and alumina particles (Table 4.1). The properties of
an epoxy resin were not measured directly, but were calculated based on the measured
value of a longitudinal sound velocity. The data for alumina powder were not tested
experimentally but they were directly taken from the referenced website (Accuratus,
2013). Both sources can be affiliated with an error and they might lead to the further error
which was carried for the further calculations. The proper estimation of mechanical
properties of an epoxy resin and alumina powder could improve the correlation between
the calculated and measured sound velocity for the described composite material.
2000
2200
2400
2600
2800
3000
3200
0% 10% 20% 30% 40% 50%
Particles volume fraction
Soun
d velocity [m
/s] Experiment
Reuss ModelDevaney ModelATACPA
59
4.6. References
Accuratus. Aluminum Oxide, Al2O3 Ceramic Properties. Website address:
http://accuratus.com/alumox.html, as of May 13, 2013.
Berryman J G. Single-scattering approximations for coefficients in Biot’s equations of
poroelasticity. J. Acoust. Soc. Amer. V.91, N2, pp. 551-571, 1992.
Chawla K K. Composite Materials: Science and Engineering. Originally published in the
series: Materials Research and Engineering 2nd ed., XIII. NY: Springer, 1998.
Devaney A J, Levine H. Effective elastic parameters of random composites. Applied
Physics Letters, V.37, N4, pp. 377-379, 1980.
Nguyen T N and Lethiecq M. Experimental Verification of the Theory of Elastic
Properties Using Scattering Approximations in (0-3) Connectivity Composite
Materials. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,
V43, N4, pp. 640-645, 1996.
Reuss, A. Berechnung der Fließgrenze von Mischkristallen auf Grund der
Plastizitätsbedingung für Einkristalle. Zeitschrift für Angewandte Mathematik und
Mechanik [Journal of Applied Mathematics and Mechanics]. V.9, pp. 49–58, 1929.
Voigt W. 1910. Lehrbuch der kristallphysik (mitausschluss der kristalloptik). Leipzig:
Teubner.
60
CHAPTER 5 Examples of Manufactured Bone Phantoms
and Their Applications
The experiments and tests performed on the developed material show that its
acoustical properties are very close to those of actual bones. This means that the material
can be successfully used for manufacturing custom made ultrasound bone phantoms for
various applications. The following chapter introduces the examples of already
manufactured phantoms and it shows the capability and potential of the developed
technology.
5.1. Ultrasound Skull Phantom
According to CHAPTER 3, the author is capable of manufacturing phantoms with
various shapes and dimensions. Furthermore, what is very important, to apply an inner
porosity structure into any location of the phantom. As an example, Figure 5.1 shows an
actual product, which is an ultrasound calvaria phantom with all desired features such as
rough and realistic topography. The phantom also contains an inner porosity structure
which follows the actual skull anatomy. Moreover, the manufacturing process does not
require highly advanced equipment like vacuum or pressure chamber, but only an oven
capable to maintain 100°C for up to a single hour. This advantage can make the material
very useful for any research group which has to do ultrasonic tests or experiments on
bones without actual bones. The developed phantom can be a good equivalent and can be
easy and quickly made in any laboratory.
61
Figure 5.1: Manufactured calvaria phantom with inner porosity layer.
The calvaria phantom presented in Figure 5.1 contains a specifically designed
various porosity structure, depending on the location of skull. Figure 5.2 shows five
distinct porosity zones within the calvaria phantom: no porosity, low porosity, low-
medium porosity, medium porosity and high porosity. Each zone, due to the porosity, has
different acoustical properties and this is particularly important from a purely research
point of view. The researcher can perform tests on different locations of the skull and
check how the inner structure affects the obtained results (i.e. accuracy, precision or just
62
quality of the image). This can be useful for i.e. improving ultrasonic transcranial
techniques for blood flow imaging or static objects detection.
Figure 5.2: Porosity zones within manufactured calvaria phantom.
Figure 5.3 shows a comparison between an actual skull bone and the
manufactured phantom. One can notice that all major features of the phantom are very
similar to the real skull and the only difference is within the level of porosity. Higher
porosity means higher ultrasonic attenuation and bigger loss of ultrasonic signal. From
the technical point of view, there is no limitation in making a phantom with high
porosity. However, in the presented case, the porosity was purposefully decreased to
provide a lower attenuation and scattering. This is useful for certain laboratory research
for the early stage of developing new ultrasonic devices. At this stage, the new prototype
of the device is in the developing process, and may not be calibrated and it may even not
work properly. The phantom with a relatively low porosity and attenuation (but still fairly
close to the realistic value) can help to improve the whole research process. The porosity
can be increased gradually for the future phantom when the device is already in a higher
stage of development.
63
Figure 5.3: Comparison between actual skull and the manufactured phantom.
The developed material with an embedded porosity structure can be successfully
used not only to manufacture a calvaria part of human skull but also much bigger parts,
such as the entire skull. From the technical point of view, there are no limitations in size
or shape of phantom and basically any bone phantom with any shape and size can be
produced using the material developed by the author. As a proof of the capability of the
presented new technology, the author formed a complete ultrasound anthropomorphic
human skull phantom which is shown in Figure 5.4. The phantom has all the required
features of actual skull including its topology, dimensions, roughness and of course all of
the acoustical properties. The inner profile of skull is also conserved which is visible on
the bottom side of Figure 5.4.
Once the full skull was created, the imagination is the only limit of features which
can positioned within the skull. Depending on the project and application one can embed
within the skull certain static objects to mimic small pieces of bones or bullets after a
head trauma or put artificial blood vessels and their abnormal versions (stenosis or
aneurysm) for ultrasonic blood flow imaging. The developed technology is very flexible
and allows manufacturing ultrasound bone phantoms for any application.
64
Figure 5.4: Completed ultrasound skull phantom made out of developed material.
5.2. Soft Tissue Phantom - An Artificial Brain
Figure 5.5 shows an artificial brain made out of polyurethane which was designed
to fit the already manufactured skull phantom. Similarly like bone phantoms mimic
properties of actual bones, the brain phantom mimics the acoustical properties of actual
brain. The properties of material used for the brain phantom and their comparison with
properties of actual brain are shown in Table 5.1. One can also notice that Table 5.1
shows the range of values for polyurethane. This means that the material can be modified
in order to increase or decrease the acoustical properties within the certain range.
Polyurethane, like other polymers, is made out of molecular chains and the length of
65
these chains can be increased or decreased by adding certain types of elastomers. The
length of molecular chains determines the hardness/softness of polyurethane which is
also directly related ultrasonic attenuation and sound speed.
Table 5.1: Properties of brain and tissue mimicking equivalent used for brain phantom.
Material Velocity [m/s] Density [g/cm3]
Attenuation [dB/cmMHz]
Acoustic Impedance [MRayl]
Source
Brain 1560 1.04 0.6 1.62 ICRU 1998 Brain phantom 1440 ± 10 ~1 0.8 ± 0.1 1.44 ± 0. 01 experiment Polyurethane 1380 – 1480* ~1 0.7 - 1.7* 1.38 - 1.48* experiment
* The acoustical properties of polyurethane can be modifiable within a range showed in the table.
66
Figure 5.5: Artificial brain for the ultrasound head phantom.
The artificial brain shown in Figure 5.5 was designed in order to meet the
requirements of the ultrasonic transcranial imaging project - a high priority project in the
research group of IDIR (Institute for Diagnostic Imaging Research). In this case the
phantom has not only a realistic topography but also various features which are as
follows:
67
• Artificial blood vessel system - It is made out of material which is acoustically
similar to the actual blood vessels. The total length of the vessels is about 1 m and
they are oriented in each direction (angle) in order to get parts of the phantom
which are easily detectable and parts which are much harder from the ultrasonic
imaging point of view.
• Stenosis - It is a location of the blood vessel where its inner diameter is much
smaller (about half or less) than the normal diameter. The phantom contains two
artificial stenosis located at the positions where they commonly appear within the
actual brain.
• Aneurysm - It is a location of the blood vessel where its inner diameter is much
larger (about twice or more) than the normal diameter. The phantom contains one
artificial aneurysm which is located at the place where it commonly appears
within the actual brain.
• Bisectional blood vessels - The type of the vessel system where one main vessel
splits into two smaller vessels and then again connects into main one. The
phantom contains one artificial bisectional blood vessel system.
• Static foreign objects embedded within the brain - The objects are made out of
small pieces of bone phantom or lead. The pieces of bone phantom mimic small
pieces of bones which can be pushed into the brain after a head trauma. The lead
pieces mimic small pieces of bullets or shrapnel which also can be pushed into
after gun headshot or an explosion (for instance during a military operation).
The head injuries may appear not only within the brain but also between the brain
and the skull bone. After a certain accident (i.e. a car accident) a human head may be
pushed back and forth by an external force. Such a force is translated to the brain which
also moves back and forth within the skull. This causes a problem because the brain is
connected with the skull by various blood vessels and these connections may be lost after
the trauma. The broken blood vessels start to bleed and the blood accumulates between
the brain and skull. Obviously, the blood cannot push out the hard skull bone, so it pushes
68
the soft brain and damages its inner structure. This is called hematoma and can result in
serious brain injury.
Presently, there is no ultrasonic device which can detect such a problem behind
the highly attenuative human skull bone. The research on such a device could be done in
the near future and the phantom which could simulate a brain hematoma could be
invaluable from the research point of view. The author of the following thesis included
such a feature in the manufactured head phantom which is visible in Figure 5.6.
Figure 5.6: Brain phantom within the skull bone phantom and the hematoma phantom attached to the brain surface.
The artificial hematoma is located on the front-right side of the created head
phantom. The hematoma phantom is basically an empty cavity which can be filled by the
blood mimicking liquid through the specifically designed tube system. Since, it is hidden
behind the bone phantom which highly matches the acoustical properties of human skull
bone, the head phantom can also be successfully used for the ultrasonic hematoma
imaging and detection.
69
5.3. Ultrasound Head Phantom
The ultrasound head phantom (Figure 5.7) consists of an artificial brain, blood
vessels and an artificial skull bone; all components mimic the acoustical properties of
their real equivalents as it was already described in previous chapters. Moreover, an
anthropomorphic shape of the phantom helps the researcher, who uses the phantom and
develops a prototype of a new device, to get familiar with the topography of an actual
head. This is obviously different than the experiments done on flat samples and it helps to
localize problems related not only to the physical properties but also to the complicated
skull profile.
The huge advantage of a manufactured head phantom is its feature to maintain the
unchanged acoustical properties for a long period of time. This is different than
experiments on ex vivo samples which, without a complicated treatment, degrades within
a couple of days. The constant and specifically designed properties allows improving the
new algorithms, methods and the new devices themselves by basically doing the
experiments and checking if obtained results are getting better or worse.
Figure 5.7: Completed full anthropomorphic human head phantom for the transcranial imaging project.
70
CHAPTER 6 Development of a Practical Ultrasonic
Approach for Simultaneous Measurement of the
Thickness and the Sound Speed in Human Skull
Bones: a Laboratory Phantom Study
6.1. Introduction
The manufactured bone phantoms (especially skull bone phantoms) can be directly
used to develop new methods of the ultrasonic human head investigation. For instance,
for the purpose of one of the high priority projects run at the Institute for Diagnostic
Imaging Research - the Ultrasonic Transcranial Imaging Project, the author used
phantoms to develop a practical ultrasonic approach for simultaneous measurement of
thickness and sound speed in human skull bone.
Non-destructive measurement of both the speed of sound and the thickness of an
object with restricted access to the opposite side is an important industrial and medical
problem and human skull presents a similar issue. The existing methods of measuring the
speed of sound in human skull bone are based on the prior knowledge of its thickness
(Geisthoff et al 2004). On the other hand, the methods of thickness measurement are
based on the knowledge of the sound velocity (Tretbar et al 2009; Hakim et al 1997;
Maev 2008). The ability to measure both parameters simultaneously and non-invasively
would open the possibility for applying automatic corrections to the ultrasonic signals
and for real-time adaptive focusing and beamforming through the skull.
To date, several methods have been proposed for simultaneously measuring the
sound speed and the sample thickness, which requires two independent equations. The
method described by (Renzel, 2008) and implemented in Krautkramer’s Auto-V
technology uses a combination of four transducers mounted on a multifaceted wedge.
71
Two of these probes are oriented obliquely to the sample to launch and receive a
longitudinal creeping wave along its surface. These probes provide the measurement of
the sound velocity. The other pair of probes sends and receives the waves through the
sample, providing data for measuring its thickness. Although this method works well in
metals, its application to the skull bone is hardly possible due to inhomogenity and
layered structure of the bone. The velocity of the surface wave propagating in the thin top
layer of the cortical bone may not be directly related to the average longitudinal sound
speed across the skull, as the latter has intermediate layers with slightly different sound
speed values. These layers (Rohen et al 1998) include the outer and inner tables
composed of cortical bone with little or no porosity, and the middle table composed of
porous trabecular bone (diploë). The diploë layer may be absent in certain cases including
newborn skulls or thin temporal bones, but it is usually present in the thickest sections of
an adult skull (Lynnerup et al 2005), which are the main focus of this work.
Another simultaneous measurement method is described in (Sinha, 2003). This
method uses swept frequency signals capable of exciting several standing wave
resonances in the sample. One equation relates sound velocity to the frequency interval
between spectral peaks corresponding to adjacent resonances. The second equation is
derived from the time of flight measurement in the temporal domain, which is obtained
by inverting the spectral data. While this method works well in industrial applications
(e.g. pipeline monitoring), its adaptation to the highly attenuative skull may require
increased power levels to excite standing waves. The necessary power level could be too
dangerous for a real biomedical application.
The first presented here method of the combined sound speed and thickness
measurement uses a single ultrasonic transducer and is based on the double focusing
technique originally described by (Hänel and Klefner, 2000). In this method, the
ultrasonic beam is successively focused on the front and back surfaces of the sample
submerged in water. The frequency used in the Hänel and Kleffner experiment was ~400
MHz, and the sample thickness was around 40 μm. This frequency is too high compared
to the frequency range that can propagate through the thick (3 to 12 mm) and highly
attenuative human skull bone. Also, the focal distance of the transducer has to be long
72
enough to be able to focus the ultrasonic wave on the back surface of the skull bone. In
our experiments, satisfactory results were obtained using a 2.25 MHz spherically focused
ultrasonic transducer with 38.1 mm focal distance and 19 mm diameter. Due to long focal
distance and low frequency, this transducer also had rather long focal zone (~13.6 mm in
water at -6dB level). As a consequence, the reflected intensity curve (V(z)) had a very
broad peak. However, it was still possible to reliably detect the maximum of that peak
and hence to determine the focal distance (see Figure 6.1).
The second presented here method for simultaneous sound speed and thickness
measurement, is an extension of the incremental focusing principle to the case of a
standard linear phase array transducer commonly used in clinical diagnostics. It is a
standalone method, which is based on a different set of equations and is better adapted to
practical applications than its single-transducer counterpart. In addition, it can be realized
with the same probe that is used for further transcranial imaging tasks requiring the
velocity and thickness information.
In contrast to the existing methods of measuring skull bone properties (e.g. X-Ray
computed tomography, ex-vivo ultrasonic measurements, etc.), the ultrasonic techniques
presented in this thesis are completely non-invasive, radiation-free, and do not require
surgical intervention.
73
Figure 6.1: Schematic illustration of the elongated beam and its intensity in the performed experiment.
For the initial validation of the algorithms, instead of testing a live human skull or
an ex-vivo skull (whose acoustical properties may differ significantly from the in-vivo
case), a series of laboratory phantoms have been developed to mimic all required physical
and acoustical parameters of the real skull. The material used for the phantoms was
described in CHAPTER 3 of the following thesis and in the paper written by Wydra and
Maev (2013).
6.2. Phantoms and Methods of Their Investigation
All the experiments were conducted on custom made skull bone phantoms designed
and developed by the author of the following thesis. The relevant physical properties of
the phantom material, such as density, speed of sound, and acoustic impedance closely
mimic those of a typical real human skull bone. For example, the speed of sound must be
in the range between 2000 m/s and 4000 m/s, depending on the density and porosity of
the skull bone (Connor et al 2002). The ultrasonic attenuation is another important
parameter, which is usually much higher in the porous sections of the skull than in those
74
containing only cortical layer. The samples used in the experiments (Figure 6.2 and
Figure 6.3) meet all above requirements and their properties are shown in Table 6.1.
Additionally, for the statistical error analysis, the 99% confidence intervals (with t critical value
2.977) were calculated for all investigated samples using Eq. 3.6.
Figure 6.2: Skull bone phantoms used in the described experiments (upper left – thin flat sample without porosity layer, upper middle – thin flat sample with porosity layer, upper right – thick flat sample, lower left – undulated sample without porosity layer, lower right – undulated sample with
porosity).
Due to its useful features and matching ultrasonic properties, this composite
material is used for manufacturing of all cortical bone phantoms in our lab. The real
cortical bone has very low porosity (around 5% in average), which is approximated by
nearly 0% porosity of the phantom material. Those rear cases when the cortical bone (in
some other areas of the human body) has high porosity (up to 30% (Carter and Spengler,
1978)) are not considered as they are not typical for the skull bone. Instead, the porosity
is localized in the diploë layer, and one of the key requirements to the skull bone
phantoms is the ability to mimic the three-layered structure. To satisfy this requirement, a
technological procedure has been developed to add inner porosity layer of any desired
thickness to the developed material, as reported in (Wydra and Maev, 2013).
75
Figure 6.3: Skull bone phantom with curved profile and inner porosity layer used for the experiments with a phased array probe.
Table 6.1: Properties of the developed skull bone phantom.
Property
Phantom Thin flat without porosity
Thin flat with
porosity
Thick flat without porosity
Undulated without porosity
Undulated with
porosity
Density [g/cm3] 2.09 2.04 2.07 2.09 2.07 Acoustic Impedance
[MRayl] 6.3 6.3 6.1 6.3 6.3
Speed of Sound [m/s] 3030 ± 21 2971 ± 21 2944 ± 21 3011 ± 27 2989 ± 10 99 % Confidence Limits
for speed [m/s] 3010 - 3050 2951 - 2991 2924 - 2964 2986 - 3036 2979 - 2998
Total Attenuation @ 2.25 MHz [dB/cm]
7.6 ± 0.4 18.8 ± 0.7 6.6 ± 0.4 9.7 ± 0.3 15.7 ± 0.7
99 % Confidence Limits for attenuation [dB/cm]
7.2 – 8.0 18.1 – 19.5 6.2 – 7.0 9.4 – 10.0 15.0 – 16.4
BUA @ 2.25 MHz [dB/cm/MHz]
3.1 ± 0.3 5.2 ±1.0 2.4 ± 0.2 2.9 ± 0.1 3.9 ± 1.5
99 % Confidence Limits for BUA [dB/cm/MHz]
2.8 – 3.4 4.2 – 6.1 2.2 – 2.6 2.8 – 3.0 2.5 – 5.3
76
In addition, the developed material can be formed to any required shape and further
modified during mechanical treatment to reflect the macro anatomical structure of the
actual skull including realistic curvature of the inner and outer tables and fine surface
topography of the inner table.
The attenuation values for the skull phantoms described in this work were in the
range 7-10 dB/cm at 2.25 MHz for cortical and 19 dB/cm at 2.25 MHz for diploë
samples. These values are realistic and close to the ex-vivo measurements reported by
(Baykov et al 2003) (14-19 dB/cm at 1.7 MHz). From technical standpoint, the
attenuation can be easily increased by adjusting the porosity level. However, the selected
level was enough to demonstrate the performance of the new technique on a realistic
skull model, while adaptation to even higher attenuation would constitute further
optimization of the technique.
The ultrasonic parameters in our samples were measured in a specially constructed
immersion test cell using both through transmission and pulse-echo methods.
The attenuation was measured using transmission mode and two flat transducers
(Technisonic, USA) at 2.25 MHz (BW @ -6dB = ~31%). The transducers were placed
coaxially, facing each other in the water tank, and the direct ultrasonic wave in water was
recorded as a reference signal. Then, the prepared phantoms were placed in water
between two transducers. The signal for each sample was recorded, and the value of the
attenuation was calculated from the following formula:
( )( ) ( )⎟⎟
⎠
⎞⎜⎜⎝
⎛−⋅= 21log20 R
fAfA
dnAttenuatio
p
w (6.1)
where Aw is the amplitude of the wave transmitted through water without sample, Apis the
amplitude of the transmitted wave when the sample is inserted. R is the reflection
coefficient:
12
12
ZZZZR
+−
= (6.2)
77
Z2 is the acoustic impedance of phantom and Z1is the acoustic impedance of water. In flat
samples, this method provides reliable and accurate attenuation measurements.
a) Single element focused transducer
In the double focusing experiment both the transducer and the skull phantom were
submerged in water, while their positions and mutual orientation were accurately
controlled by the X-SEL 4-axis scanner (IAI, Japan).
To achieve optimal results it is critical to choose a transducer with appropriate
frequency, dimensions and surface curvature. In our experiment we used a 2.25 MHz
ultrasonic transducer (Technisonic, USA) with 19 mm-diameter and 38.1 mm nominal
focal distance in water (properties in Table 6.2). The general principle of the double
focusing experiment is presented in Figure 6.4. The measurement involves focusing the
acoustic field on the front and back surfaces of the sample and accurately measuring the
corresponding transducer positions. The experimental setup is schematically shown in
Figure 6.5. The ultrasonic transducer driven by broadband pulses from UT340 pulser-
receiver (UTEX Scientific Instruments, Mississauga, Ontario, Canada) was incrementally
(at 0.25 mm steps) moved towards the sample, and the entire reflected waveform was
recorded at each position using ADM1614x65M 14-bit, 65Mss data acquisition board
(Instrumental Systems, Moscow, Russia). The peak intensities of the front and back
surface echoes were measured and plotted vs. transducer position using LabView
(National Instruments, USA) software (Figure 6.6). These curves exhibit maxima when
the ultrasonic wave is focused on the front and back surfaces of the sample respectively.
Since the focal zone of the transducer is elongated (as explained in the Introduction and
in Figure 6.1), the maxima of both curves are relatively flat. Nevertheless, it is still
possible to accurately locate them and then determine the transducer positions
corresponding to focusing of the ultrasonic beam on the front and back surfaces.
78
Figure 6.4: The principle of the double focus experiment (left – position of the sample and transducer, right – recorded A-Scans from both positions).
Table 6.2: Properties of the focused single element transducer used in the performed experiment.
Peak Freq (MHz) 1.953 Center Freq (MHz) 2.002 -6dB BW (MHz) 2.051
-6dB BW (%) 102.448 -6dB Lo (MHz) 0.977 -6dB Hi (MHz) 3.027
79
Figure 6.5: Schematic configuration of the experimental set-up for the single point measurement.
Figure 6.6: Recorded A-Scan for the thin flat phantom without porosity. During the double focus experiment the reflections and their amplitudes from the front and back surfaces were tracked (upper graph), and the distance between transducer positions corresponding to the recorded
maxima was calculated (lower graph).
Using the symbols defined in Figure 6.4, the thickness and the speed of sound can be
obtained from the following equations (Hänel and Klefner, 2000):
80
212
1 tcABd == (6.3)
102
1'' tcACCA == (6.4)
10
21
tctc
ACAB
= (6.5)
where d is the sample thickness, 1t is the delay of the front surface echo when the
transducer is focused on it, 2t is the delay between the front and back surface echoes, 0c
and 1c are the speed of sound values in the surrounding medium and in the sample, the
angles α and β are defined in Figure 6.4.
From the geometrical relations one can write:
ACAB
=βα
tantan
(6.6)
From the Snell’s law:
1
0
sinsin
cc
=βα
(6.7)
Combining Eq. (6.3) - (6.7) one can obtain the expression for the speed of sound in the
sample:
21
20
22
2
1
2
tan
tct
c Δ+−=
α (6.8)
where
αα 22
1
224
cos4tan
tt
+=Δ (6.9)
The thickness can then be calculated by substituting (6.8) into (6.3).
81
b) Linear phased array probe
The incremental focusing method can be modified to work with a phased array
transducer. In this case, instead of changing the position of a single transducer with
permanent focal distance, the focal distance is changed electronically, while the multi-
element transducer remains in place.
The idea of using an ultrasonic transducer array for the simultaneous measurement
of the thickness and the sound velocity was previously described by (Titov et al 2009).
The proposed method assumed submerging the sample in water and measuring the
desired parameters by transmitting the ultrasonic wave with one single element of the
probe and receiving it with all elements sequentially. This method allows measuring the
longitudinal and the transversal velocity as well. However, one single array element is
not enough (due to its small size and no focusing) if one considers such highly
attenuative and thick medium as the human skull bone. To increase the transmitted signal
power, the proposed here method implements focusing of the ultrasonic wave at a single
point inside the sample using multiple array elements. This technique allows achieving
much better signal to noise ratio.
The algorithm of this procedure looks as follows. First, the phased array is
spherically focused at some depth within the sample using a realistic guess for the sound
speed. The guessed value can be quite arbitrary, with the only requirement that the
resulting focal point falls somewhere between the sample’s boundaries. The timing
pattern is organized so that the delays of the end elements are equal to zero, and the delay
of the central element is maximal. Then, the timing pattern is changed, so that the array is
dynamically focused to find the back surface of the sample. In this case, the focal
distance is continuously increased until the highest amplitude of the reflection from the
back surface is obtained. The focal distance is specified by prescribing user-defined time
delays for 64 elements of the probe. The iteratively found timing pattern corresponding to
the focal distance at which the back surface echo reaches its maximum amplitude is
recorded for further use. The most important for further analysis is the value Δt1 of the
time delay of the central element with respect to the delay of the end elements, which is
82
kept equal to zero during dynamic focusing. Based on these measurements and Figure
6.7describing the experiment, the following equations can be written:
cdT 2
=Δ (6.10)
ctdAF ⋅Δ+= 1 (6.11)
22 daAF += (6.12)
where Δt1 is the time delay of the central element of the phased array with respect to the
(zero) delays of end elements when the array is focused at the back surface; ΔT is the
time delay of the back surface echo measured from the A-Scan corresponding to the
focus at the back surface, a is the half-length of the array; d is the sample thickness.
Combining (6.10)-(6.12) yields two final equations for the sound speed c and thickness d
of the sample:
Ttad
ΔΔ
=+
= 12;)2(
ζζζ
(6.13)
Tdc
Δ=
2 (6.14)
The implementation of the incremental focusing principle on a phased array probe is
shown in Figure 6.7 and Figure 6.8. Compared to the single transducer case, this
technique does not require immersion and does not include mechanically moving parts. It
should be mentioned that inserting an ultrasonic delay pad between the transducer and the
skull would allow to retain the original equations (6.3) and (6.8) and also to focus the
ultrasound onto a smaller area. However, such configuration is incompatible with our
adaptive beamforming algorithms, which require direct contact with the head. So, in
practice the operator would have to first use the delay pad to measure the skull profile,
then remove it and acquire a transcranial image. Instead, our approach based on Eq.
(6.13) and (6.14) allows for uninterrupted freehand imaging without worrying about
properly repositioning the transducer each time.
83
The experimental setup is shown in Figure 6.8 and Figure 6.9. The Ultrasound
Advanced Open Platform (ULA-OP) phased array controller (Universita di Firenze,
Florence, Italy) with 64 independent transmit-receive channels was connected to a 128-
element linear array probe (ESAOTE, Italy), and the frequency was programmatically set
to 2.25 MHz. According to the above theory, the equipment has to be able to focus
ultrasonic beam at multiple depths with a relatively small increment (e.g. ~250 μm as in
the single-transducer experiment). To achieve this goal, a set of 48 delay patterns for 64
array elements was designed to provide all desired focal points within the 3 - 15 mm
depth interval (thickness of the human skull) for the investigated phantoms. The delay
patterns were successively applied to the phased array transducer in an automated fashion
(Figure 6.10).
Figure 6.7: Using a flat, linear phased array transducer for measuring the thickness and the sound velocity.
Figure 6.8: Schematic configuration of the experimental set-up for the phased array transducer.
84
Figure 6.9: The curved porous phantom and the phased array probe during the performed experiment.
Figure 6.10: Time delay patterns for the phased array probe for 7 out of 48 programmed focal points (3mm, 5mm, 7mm and so on).
85
6.3. Results from the Experimental Work
The single transducer method was tested on three flat samples (two thin and one
thick) with and without porosity layer (Figure 6.2), and on two samples having one flat
and one undulated surface (Figure 6.2), with and without porosity layer. The phased array
method was tested on two flat (thick and thin) samples without porosity (Figure 6.2) and
on the curved skull phantom with a porosity layer (Figure 6.3).
6.3.1. Measurements Using a Single-Element Focused Transducer
a) Thick flat phantom without porosity
The thickness and the velocity were measured at four different points of the sample,
and at each point the scanning was repeated 10 times. The obtained results summarized in
Table 6.3 can be used to estimate the feasibility of the described method. The data shows
that the difference between calculated average thickness values obtained from the
experiment and using the caliper is not bigger than ~2.5 % in the worst case scenario and
~0.5 % in the best case scenario, which for the investigated sample is equal to 0.27 mm
and 0.05 mm respectively.
The measurement of the speed of sound in the same sample shows that the
difference between the data obtained from the double focusing experiment and from the
standard ultrasonic transmission method (two flat transducers connected to the sample) is
equal to 1.59 % and 0.03 % for the worst and best case scenarios (Table 6.4) which is
even smaller relative error than for the thickness measurement.
b) Thin flat phantom without porosity
The experiment was carried out the same way as above but at this time on a thinner
sample (Figure 6.2). In this case the measurement was repeated 10 times over one sample
point. The averaged value of the thickness obtained from the experiment is 7.00 mm. The
thickness obtained by caliper is 7.21 mm. The difference between these two values is
0.21 mm, and the relative error is ~2.9 %.
86
The speed of sound measured by the above technique is 2912 m/s and by the
standard transmission method is 3004 m/s. The difference is 92 m/s which give the
relative error equal to ~3.1 % (Table 6.5).
c) Thin flat phantom with porosity layer
A flat bone phantom with a single porosity layer (Figure 6.2) was investigated in
the same way and using the same equipment as before. One point of the sample was
chosen, and the thickness and the sound velocity were measured 10 times. The example
of a single recorded A-Scan is shown in Figure 6.11.
Figure 6.11: An example A-Scan recorded on the bone phantom with a single porosity (diploë) layer.
Table 6.3: The thickness of the flat thick phantom calculated from the double focused experimental data and its comparison with the thickness measured by caliper.
Point of measurement (Figure 6.2)
N
Averaged calculated thickness
ACT [mm]
SD [mm]
Thickness measured by caliper TMC
[mm]
Difference
TMC-ACT [mm]
Δ [%]
99% Confidence Interval for the
ultrasonic experiment [mm]
A 10 10.61 0.11 10.83 ± 0.05 0.22 ± 0.12 2.03 10.50 – 10.71
B 10 10.82 0.08 10.90 ± 0.05 0.08 ± 0.09 0.73 10.74 – 10.89
C 10 10.58 0.14 10.85 ± 0.05 0.27 ± 0.15 2.49 10.45 – 10.71
D 10 10.86 0.09 10.81 ± 0.05 0.05 ± 0.10 0.46 10.77 – 10.94
87
Table 6.4: The speed of sound in the flat thick phantom calculated from the double focus experimental data (SOSE) and its comparison with the speed of sound measured by transmission method with prior knowledge of the sample thickness (SOST).
Point of measurement
N
Averaged calculated
SOSE [m/s]
SD [m/s]
SOST [m/s]
Δ Difference SOSE-SOST
[m/s] Δ [%]
99% Confidence Interval for the
ultrasonic experiment [m/s]
A 10 2910 30 2927 ± 21 17 ± 37 0.58 2882 - 2938
B 10 2961 23 2962 ± 21 1 ± 31 0.03 2939 - 2983
C 10 2917 38 2964 ± 21 47 ± 43 1.59 2881 - 2953
D 10 2967 25 2922 ± 21 45 ± 33 1.54 2943 - 2990
Table 6.5: The thickness and the sound speed of the flat thin phantom without porosity layer obtained from the experiment (averaged value from 10 points) and from an independent reference method.
Thickness [mm]
Velocity [m/s] 99% Confidence Interval [mm]
99% Confidence Interval [m/s]
Experiment 7.00 ± 0.11 2912 ± 48 6.90 – 7.10 2867 - 2957 Reference 7.21 ± 0.05 3004 ± 35 7.16 – 7.26 2971 - 3037 Difference 0.21 ± 0.12 92 ± 60 - -
% Difference 2.91 3.06 - -
Table 6.6: The thickness and the sound speed of the flat thin phantom with porosity layer obtained from the experiment (averaged value from 10 points) and from an independent reference method.
Thickness [mm]
Velocity [m/s] 99% Confidence Interval [mm]
99% Confidence Interval [m/s]
Experiment 6.39 ± 0.15 2861 ± 65 6.25 – 6.53 2780 - 2922 Reference 6.98 ± 0.05 2983 ± 35 6.93 – 7.03 2950 - 3016 Difference 0.59 ± 0.16 122 ± 74 - -
% Difference 8.45 4.09 - -
The averaged values obtained from the performed experiment are presented in Table 6.6.
According to the data, the precision of the measurement is 0.6 mm for the thickness
and 122 m/s for the sound velocity which give relative errors of ~8.5% and 4.1%
respectively for both parameters.
88
These values are about twice as large as those for the sample without porosity layer.
In this case the measurement was affected by two main factors. Firstly, the ultrasonic
wave is refracted not only at the water-phantom boundary but also on the pores inside the
phantom. It means that the previous calculations based on simple geometrical relations
can be treated only as an approximation. Secondly, reverberation from the porosity layer
can overlap with the back surface echo making it difficult to identify the latter on the A-
Scan.
Figure 6.12: (a) – Thickness profile measured 4 times for the same sample; (b)– Averaged thickness profile of the undulated sample obtained with the double focus method superimposed on the actual thickness profile; (c) – Ultrasonic B-Scan of the same specimen; (d) – The profile
obtained from the B-Scan and the speed of sound measured at a single zero-slope point.
89
Figure 6.13: Reflections of an obliquely incident focused beam at two different transducer positions: more ultrasonic energy is captured by the transducer in the defocused position.
Figure 6.14: a) – Ultrasonic B-Scan of the sample with inner porosity layer; b) Ultrasonic B-Scan with the sound speed correction obtained from the experiment and the actual superimposed
sample profile in the background.
90
d) Flat - undulated phantom without porosity
The double-focusing method was also tested on a specimen with flat outer
(transducer side) surface and curved inner one. The surface curvature was purposely
made one-dimensional to accommodate cylindrical symmetry of the ultrasonic field
produced by the array elements (long thin strips). The goal of this test is to understand the
effect of non-parallel surfaces on the results.
To test the reproducibility and precision of the presented method, the thickness
measurement throughout the sample (profile) was repeated four times (each time at a
slightly different area due to transducer positioning inaccuracy), and the obtained results
are presented in Figure 6.12.a. The measured profiles match rather well (their main
features are at the same locations) but there are subtle variations between them. This
could be due to the mentioned slight shift in position and to the limited precision of the
method itself which is about 0.3 mm (according to the data presented in the previous
sections for flat samples). The origin of this noise is in the precision of the data extraction
algorithm, which also depends on the fluctuations in the signal amplitude and shaking of
the transducer and the phantom, so that even the repeated measurement at the same point
for a flat sample will not produce identical values each time. This noise can be
considerably decreased by averaging over multiple acoustic measurements at each point.
In Figure 6.12.b, the ultrasonically measured thickness (averaged data from four
trials presented in Figure 6.12.a) with double focusing method were superimposed on the
actual profile of the sample. It can be seen that the error is minimal at the points where
the inner and outer surfaces are locally parallel, but it becomes significant when the
parallelism is violated. Indeed, equations 8 and 9 are valid only at the points where inner
and outer surfaces are locally parallel. As shown in Figure 6.13, when the inner surface of
the sample is tilted with respect to the outer one, the energy is partially deflected away
from the transducer, and the maximum echo is achieved when the transducer is closer to
the reflecting surface than its focal length. This difference between the assumed and the
actual positions of the maximum leads to the lower thickness measurement accuracy at
the points of increasing angle between the inner and outer surfaces.
91
To remove the ‘geometrical component’ of the error, the unknown angle between
the front and back surfaces can be obtained from the B-Scan representation (Figure
6.12.c), which is a part of the data set recorded during the double focusing experiment.
Once the sample profile is extracted from the B-Scan (e.g. using either pulse tracking or
edge detection algorithms), it is easy to find points where the surfaces are locally parallel
and apply the incremental focusing method there. Then, knowing the sound speed
measured at those points, the entire B-Scan can be converted from the time delay to
thickness units (Figure 6.12.d).
e) Flat - undulated phantom with porosity
Here the phantom with one flat and one undulated surface had an inner porosity layer
(see Figure 6.2). The obtained results are presented in Figure 6.14.a and b. Figure 6.14.a
presents a raw B-Scan of this specimen, which allows finding a zero-slope point on the
back surface. After measuring the sound velocity exactly at that spot, the B-Scan was
converted from time delay to thickness units, as shown in Figure 6.14.b.
It should be noted that the above approach is strictly valid only for homogeneous
samples where the sound speed does not change from point to point. In practice,
heterogeneity of the skull bone will lead to the errors in the skull thickness obtained with
Eq. 8 and 9 under assumption of constant sound speed. As a counter argument, however,
the local variations of the sound speed within the same skull rarely exceed 150 m/s (5%)
which was noted by White et al. 2006 (3100 m/s and 3250 m/s for two points of the skull
distanced by ~1 cm). Therefore, the suggested approach based on the single point
measurement of the real sound speed is still better than using some literature-based sound
speed value from a different skull.
6.3.2. Measurements Using a Linear Phased Array Probe
a) Thick and thin flat phantoms without porosity layer
The experiment was performed on the two flat phantoms shown in Figure 6.2: the
thin phantom with nominal thickness 7.2 mm and the thick one with thickness 10.9 mm.
The example of the recorded A-Scans corresponding to selected 6 designed delay patterns
92
for the thin phantom are shown in Figure 6.15. Each line on the graph represents an A-
Scan from a focal point at a particular depth of the investigated sample. The highest
amplitude A-Scan represents the case when the ultrasonic beam is focused on the back
surface of the phantom. The time of flight ΔT for this scenario and the delay Δt1 from the
corresponding timing pattern were substituted into equations (6.13, 6.14).
The thickness of the “thick” flat phantom (Figure 6.2) measured with the phased
array method was 10.86 mm (~0.4 % relative error). For the “thin” phantom, the same
method provided thickness estimation 7.55 mm (~4.9% error), which is much less
accurate than for the “thick” sample.
b) Curved phantom with inner porosity layer
Based on the promising results with the flat phantoms, the experiment was also
conducted on the phantom with a realistic curved 3D profile (Figure 6.3), which is very
similar to the profile of an actual human skull bone. Moreover, the phantom has inner
porosity layer to even closer mimic the reality and to avoid in-vivo testing or using
cadaver samples at this proof-of-concept stage.
In this experiment, the phantom and the phased array probe were submerged in
water, and the probe was arranged exactly perpendicular to the phantom surface to
minimize the error. The measurement was repeated at three different spots on the
phantom. The spots were selected to have the front and the back surfaces as locally
parallel as possible. The obtained results were compared with the actual thickness
measured by caliper and the data are presented in Table 6.7.
It can be noticed that the measured thickness values for all three spots are very close
to the actual thickness measured with the caliper, and the error is not bigger than 3.7 %.
The biggest error occurred for the thinnest place, which is in agreement with the results
from the flat samples. This experiment also showed that the porosity layer did not have a
detrimental effect on the measurement accuracy. However, it still affected the accuracy to
some extent. Indeed, the porosity within the manufactured phantom is not perfectly
homogenous, which could be the reason for the difference in precision at points A and C.
93
Figure 6.15: Example of 6 out of 48 A-Scans obtained with the phased array transducer for the thin flat phantom without porosity layer (each line represents a different focal point: ~3mm,
~5mm, ~7mm and so on).
Table 6.7: Results obtained from phased array probe and curved 3D skull phantom with inner porosity layer.
Spot Thickness (caliper)
[mm]
Thickness (experiment)
[mm]
Difference [mm]
Difference [%]
A 8.2 ± 0.1 8.5 0.3 3.7 B 9.0 ± 0.1 8.7 0.3 3.3 C 8.4 ± 0.1 8.5 0.1 1.2
* Since the investigated skull bone phantom had a complicated topography, it was practically not possible to locate the ultrasonic probe several times at the same spot; the results come only from a single point
measurement and the statistical analysis was not possible.
6.4. Discussion
The errors associated with all measured parameters were calculated according to the
well known rules of statistical analysis. The author calculated Standard Deviation and
99% Confidence Intervals for all investigated parameters to show a certainty level for the
developed method. Additionally, all results from the ultrasonic experiments were
94
compared with the results obtained from different independent methods (i.e. using a
caliper to measure a sample thickness).
The experiments conducted on flat samples demonstrate the accuracy of the single
point measurement. According to the obtained results, the thickness measured using the
described method with a single-element transducer on the thick sample differs from the
value obtained from the caliper by 0.27 mm (2.5 %) in the worst case scenario. The
relative error is slightly bigger for the thin phantom (2.9 %), but the difference is in the
range of statistical error.
On the other hand, the relative error of the measured value of the sound velocity is
about two times smaller (~1.5 %) for the thick sample than for the thin one (~3 %).
Hence the precision of the speed of sound measurement can possibly depend on the
thickness of the sample, and to confirm this prediction it is necessary to do more
experimental work and data analysis. The errors could also be reduced by implementing a
more sophisticated echo tracking algorithm than the simple threshold-based one used
here to prove the concept.
The results obtained from the phantom with a thin internal porosity (diploë) layer
are showing that for the described technique the precision of the measurement is about
~2.5 times lower than for the phantom without any porosity. In this scenario the precision
of the thickness and speed of sound measurement is about 8.5 % and 4.1 % respectively.
The accuracy of the measurement on the undulated phantom depends on the
curvature of the sample, and the error increases with the angle between the inner and
outer surfaces (Figure 6.12.b). However, if the sample profile is known (for example,
from the raw B-Scan data as in Figure 6.12.c and Figure 6.14.a), then it is possible to find
a location where the surfaces are locally parallel, and to make the thickness-velocity
measurement exactly at that point. This algorithm has provided a satisfactory profile of
the investigated skull bone phantom with 0.1 mm average accuracy (Figure 6.12.d and
Figure 6.14.b).
Based on the experimental results obtained at 2.25 MHz it should be noted that the
skull thickness measurable with the described algorithm cannot be lower than 2.4 mm. In
95
other words, the reflections from the front and back surfaces must be clearly separated
from each other to be detected by the developed algorithm. The same case is with the
inner porosity layer – if the reflections from pores distort the reflection from the back
surface (due to overlapping) it is hard to find the back profile of the sample.
The adaptation of the incremental focusing technique to the phased array transducer
has demonstrated faster operation speed and easier application, as this method does not
require mechanically moving parts and immersion in water. In addition, the data
extraction procedure is much simpler and less prone to error than in the immersion case.
Moreover, the obtained results show that the precision of the measurement for the
phased array transducer is comparable or even better than the precision achieved with a
single element transducer.
The lowest observed measurement precision for the phased array occurred for the
thin phantom which could be related to using a relatively wide aperture (64 elements ≈ 10
mm) for focusing at too close distances to the probe. In such case the contribution of edge
elements at the focal point would be disproportionately small (due to their limited
directivity) and lead to possible distortion of the focal point. Hence, for focusing at short
distances it is recommended to use fewer array elements and then increase their number
as the depth increases.
Nevertheless, all performed experiments show that the precision of the
measurement is comparable to the defocusing step size of 0.25 mm (same step for the
single transducer and phased array) and is no worse than 0.35 mm (also for the curved 3D
phantom with inner porosity layer). This value is even smaller than the ultrasonic
wavelength in skull bone which is equal to ~1.4 mm at 2.25 MHz. However, the
wavelength is still the parameter which limits the maximum resolution of the presented
methods.
For the case when one assumes non-parallel surfaces, when the angle between outer
and inner surface vary, the same correction approach can be used as in the single-
transducer case. First, the array can be configured to produce a B-Scan of the sample. The
extracted slope values can then be used to find a zero-slope point where the above
96
described dynamic focusing algorithm can be applied without any changes using either
all or a subset of the array elements. A proper setup of this experiment requires a matrix
(rather than linear) phased array and goes beyond the scope of this article.
The successful results obtained with phased array can be directly applied to the
adaptive beamforming procedure used by our team (Shapoori et al 2010) to focus the
ultrasonic beam through a layered bone structure. It should be also noted that the skull
surface is not perfectly smooth and usually more or less rough (especially the outer
surface). This could be another source of error but so far the experiments performed on
the realistic phantom (Figure 6.3) did not show any significant impact on the precision of
measurement for the presented method with a phased array probe. However, the authors
are aware that an in vivo study has to be conducted to fully confirm the practical
applicability of the presented results.
6.5. Conclusions
It has been demonstrated that the simultaneous measurement of the speed of sound
and the thickness of human skull bone phantoms is practically possible, and both
parameters can be estimated with the satisfactory precision (up to 3 %). The two methods
were proposed and experimentally tested: one using a single focused transducer and the
other one - a linear phased array probe. The second method is more attractive from the
practical standpoint as it has a simpler procedure and does not require scanning. The big
advantage of the proposed ultrasonic techniques is their non-invasive character. In
addition, both methods are self contained and do not require indirect measurements, such
as Lamb or Rayleigh waves along the bone. The developed methods and algorithms can
also be applied to the real human skull bone, and the in vivo experiments will be the next
step of the research in our laboratory.
6.6. References
Ammi AY, T. Mast TD, Huang IH, A. Abruzzo TA. Coussios CC, Shaw GJ and Holland
CK. Characterization of Ultrasound Propagation Through Ex Vivo Human Temporal
Bone. Ultrasound in Med. & Biol. V.34, N10, pp. 1578-1589, 2008.
97
Aubry JF, Tanter M, Pernot M, Thomas JL, and Fink M. Experimental demonstration of
noninvasive transskull adaptive focusing based on prior computed tomography scans.
J. Acoust. Soc. Am. V.113, N1, pp. 84-93, 2003.
Baykov SV, Babin LV, Molotilov AM, Neiman SI, Riman VV, Svet VD, and Selyanin
AI. Physical and Technical Aspects of Ultrasonic Brain Imaging through Thick Skull
Bones: 2. Experimental Studies. Acoustical Physics. V.49, N4, pp. 389–395, 2003.
Carter DR, Spengler DM. Mechanical properties and composition of cortical bone.
Clinical Orthopaedics and Related Research. V.135, pp. 192–217, 1978.
Connor CW, Clement GT and Hynynen K. A unified model for the speed of sound in
cranial bone on genetic algorithm optimization. Physics in Medicine and Biology.
V.47, pp. 3925-3944, 2002.
Fry FJ, Barger JE. Acoustical properties of the human skull. J. Acoust. Soc. Am. V.5,
N63, pp. 1576-1590, 1978.
Geisthoff UW, Tretbar SH, Federspil PhA, Plinkert PK. Improved ultrasound-based
navigation for robotic drilling at the lateral skull base. International Congress Series.
V.1268, pp. 662-666, 2004.
Hakim S, Watkin KL, Elahi MM, Lessard L. A New Predictive Ultrasound Modality Of
Cranial Bone Thickness. Ultrasonics Symposium, 1997. Proceedings., IEEE. V.2, pp.
1153-1156, 1997.
Hänel V and Klefner B. Double focus technique for simultaneous measurement of sound
velocity and thickness of thin samples using time-resolved acoustic microscopy.
Acoustical Imaging. V.24, pp. 187-192, 2000.
Lynnerup N, Astrup JG and Sejrsen B. Thickness of the human cranial diploë in relation
to age, sex and general body build. Head & Face Medicine. V.1, N13, pp. 1-22, 2005.
Maev RG. 2008. Scanning Acoustic Microscopy. Theory and Applications, Manuscript.
Publisher: John Willey and Son-VCH, approx. 450.
98
Pichardo S, Sin VW and Hynynen K. Multi-frequency characterization of the speed of
sound and attenuation coefficient for longitudinal transmission of freshly excised
human skulls. Physics in Medicine and Bioliogy. V.56, pp. 219–250, 2011.
Pinton G, Aubry JF, Bossy E, Muller M, Pernot M,and Tanter M. Attenuation, scattering,
and absorption of ultrasound in the skull bone. Med. Phys. V.39, N1, pp. 299-307,
2012.
Renzel P. Method for determining the sound velocity in a basic material, particularly for
measuring the thickness of a wall. US pat. 7,415,880, Aug. 26, 2008.
Rohen JW, Yokochi Ch, Lutien-Drecoll E. 1998. Color Atlas of Anatomy. Fourth
Edition, by F. K. SchattauerVerlagsgesellschaftmbH. Stuttgart: Williams & Wilkins.
Shapoori K, Sadler J, Malyarenko E,Severin F; Boni E,Ramalli, A, Tortoli P,Maev
RG.Adaptive beamforming for ultrasonic phased array focusing through layered
structures. Ultrasonics Symposium (IUS), 2010 IEEE, pp. 1821 – 1824.
Sinha D. Noninvasive characterization of a flowing multiphase fluid using ultrasonic
interferometry. US Pat. 6.644,119 B1, Nov.11, 2003.
Titov SA, Maev RG and Bogachenkov AN. Measuring the Acoustic Wave Velocity and
Sample Thickness Using and Ultrasonic Transducer Array. Technical Physics Letters.
V.35, N11, pp. 1029-1031, 2009.
Tretbar SH, Plinkert PK and Federespil PA. Accuracy of Ultrasound Measurements for
Skull Bone Thickness Using Coded Signals. IEEE Transactions on Biomedical
Engineering. V.56, N3, pp. 733-40, 2009.
White PJ, Clement GT, and Hynynen K. Longitudinal and Shear Mode Ultrasound
Propagation in Human Skull Bone. Ultrasound in Med. &Biol. V.32, N7, pp. 1085–
1096, 2006.
Wydra A, Maev RG. The novel composite material specially developed for the
ultrasound bone phantoms: cortical, trabecular and skull. [Submitted to: Physics in
Medicine and Biology in May 2013].
99
CHAPTER 7 Final Conclusions and Future Work
7.1. Final Conclusions
This thesis introduced a recently developed technology for manufacturing a novel
composite material for making ultrasound bone phantoms that highly matches the
acoustical properties of actual bones. The thesis presented the properties of the material,
methods of its manufacturing, investigation and its possible biomedical applications. The
thesis proved that manufactured phantoms maintain the acoustical properties
corresponding to the tissue equivalents and it proved that developed phantoms can be
successfully used for various experiments and tests instead of using ex vivo or even in
vivo samples.
The ultrasound head phantom that was presented in the previous chapter was
introduced as a final product of the research done by the author. It is a professional
product which can be commercialized in the near future and it can be used by other
research groups not only at the University of Windsor but also at other research facilities.
It should be also noted that the manufactured phantoms are not implants and in
the current state cannot be used for any surgical operation to replace the living tissues in
human body. The phantoms were made out of material which would degrade within the
human body and it would become toxic for the surrounding cells. As well the phantoms
do not have the mechanical properties of living tissues but only the acoustical one. The
microstructure of actual bones also differs from microstructure of phantoms but it was
experimentally proven that this does not affect the realistic acoustical properties of
phantoms. The author states that all presented phantoms were designed only for the
laboratory research purposes. They are already used to test and evaluate prototypes of
devices in our lab. In the future, if they get approved and certified by certain institutions
(i.e. FDA, IRCU), they can be used to test new medical ultrasonic devices and to learn
how to use them in a proper way before doing experiments on actual human body.
100
7.2. Future Work
The research done by the author allows for manufacturing various phantoms in a
quick and easy way and for a relatively cheap price. This technology can be very useful
for the transcranial blood flow imaging project run at The Institute for Diagnostic
Imaging Research. Currently, the project requires a specific tool to test and to calibrate a
prototype of the new ultrasonic device. As a solution, the author proposes to use a
developed technology to manufacture a specifically designed calibration phantom
according to the design shown in Figure 7.1. The phantom can have four distinct skull
bone phantoms (made out of the developed material) on its top with four different
porosity levels: no porosity, low porosity, medium porosity and high porosity. Each
sample will have the same specifically designed topography with a flat outer surface and
a 1D sinusoidal inner surface. Moreover, the phantom will have six tubes made out of
material with similar acoustical properties like the properties of actual blood vessels. The
tubes will be divided into two groups: the first group can be located 3cm underneath the
skull; the second group can be located twice as deep as the first one. For the calibration
purposes and to check the resolution of the new ultrasonic device, the tubes within each
group will have three different diameters but a constant wall thickness. All features (bone
phantoms and tubing) will be embedded within a soft tissue mimicking material. The
described above work can be done within two months and it is a very near future plan
which can be easily done based on the research described in this thesis.
101
Figure 7.1: Calibration phantom for the transcranial blood flow imaging project
The developed technology of bone phantom manufacturing can be also used for
the future for making an osteoporotic bone phantom. Osteoporosis is a disease of the
bones which changes the structure of bones and affects their mechanical and physical
properties. The phantom which could mimic those changes would be an invaluable tool
for any ultrasonic device for the investigation of osteoporosis and any bone assessment.
The following thesis already described a novel composite material which mimics the
acoustical properties of actual cortical bones. It also introduced an easy and quick way of
adding small particles into the material and making a simply porosity structure which
acoustically behaves similarly to actual trabecular bones. Moreover, created in this way
trabecular bone phantoms can be specifically designed in order to get the right level of
porosity or even the right size of pores. This opens an easy way for making phantoms not
only of healthy bones but also of bones affected by osteoporosis. Moreover, the phantoms
can be molded to any shape which is a very important feature if one wants to fit the
phantom to the particular device. For instance, considering current ultrasonic devices
used for bone assessment, most of them do heel bone and the phantom can be molded to
that shape. Moreover, the heel bone phantom can be surrounded by a soft tissue
mimicking material in order to make the phantom of the entire foot. All the technology
102
for the described above phantom is already developed and such a phantom can be done
within a few months.
In order to improve the described forming process of the ultrasound bone
phantom, the author sees a great potential in 3D printing technology. Currently, the
described in thesis technology of phantom manufacturing based on mold making and
pouring the proper material into it. In the future the phantom would not need to be
created using molds. Instead of that, the material could be just loaded into the 3D printer
and directly printed to any desired shape. For right now, this may sound like a script from
a sci-fi movie, but the author already sees the way of developing such a technology.
There is currently a bunch of various thermoplastics available with various properties.
The issue is that none of them maintain the acoustical properties of bones. However, by
using a similar technology like for the already developed phantoms, which means by
adding small particles into the thermoplastic material, one can modify its properties in
order to get those similar to the properties of bones. Even now, the author already
maintains a knowledge of modifying the previously used epoxy resin into a thermoplastic
epoxy resin without significantly changing its acoustical properties. Once a thermoplastic
material possess the desired properties, the way of loading it into the FDM 3D printer and
direct printing the ultrasound bone phantoms becomes a very straight forward procedure.
The research on that new technology can be the next step of the work started by the
author of the following thesis.
103
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thesis/dissertation Development of a new forming process to fabricate a wide range of phantoms that highly match the acoustical properties of human bone Expected completion date Jun 2013 Estimated size (number of pages) 125 Elsevier VAT number GB 494 6272 12 Permissions price 0.00 USD VAT/Local Sales Tax 0.0 USD / 0.0 GBP Total 0.00 USD ELSEVIER LICENSE TERMS AND CONDITIONS May 31, 2013 This is a License Agreement between Adrian Wydra ("You") and Elsevier ("Elsevier") provided by Copyright Clearance Center ("CCC"). The license consists of your order details, the terms and conditions provided by Elsevier, and the payment terms and conditions. All payments must be made in full to CCC. For payment instructions, please see information listed at the bottom of this form. Supplier Elsevier Limited The Boulevard,Langford Lane Kidlington,Oxford,OX5 1GB,UK Registered Company Number 1982084 Customer name Adrian Wydra Customer address 688 University Ave West Windsor, ON N9A 5R5 License number 3159141506584 License date May 31, 2013 Licensed content publisher Elsevier Licensed content publication Annals of Anatomy - Anatomischer Anzeiger Licensed content title Clinical anatomy of the calcaneal tuberosity Licensed content author David Kachlik,Vaclav Baca,Martin Cepelik,Premysl Hajek,Vaclav Mandys,Vladimir Musil Licensed content date 11 June 2008 Licensed content volume number 190 Licensed content issue number 3 Number of pages 8 Start Page 284 End Page 291 Type of Use reuse in a thesis/dissertation Intended publisher of new work other Portion figures/tables/illustrations Number of figures/tables/illustrations 1
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Format both print and electronic Are you the author of this Elsevier article? No Will you be translating? No Order reference number Title of your thesis/dissertation Development of a new forming process to fabricate a wide range of phantoms that highly match the acoustical properties of human bone Expected completion date Jun 2013 Estimated size (number of pages) 125 Elsevier VAT number GB 494 6272 12 Permissions price 0.00 USD VAT/Local Sales Tax 0.0 USD / 0.0 GBP Total 0.00 USD ELSEVIER LICENSE TERMS AND CONDITIONS May 31, 2013 This is a License Agreement between Adrian Wydra ("You") and Elsevier ("Elsevier") provided by Copyright Clearance Center ("CCC"). The license consists of your order details, the terms and conditions provided by Elsevier, and the payment terms and conditions. All payments must be made in full to CCC. For payment instructions, please see information listed at the bottom of this form. Supplier Elsevier Limited The Boulevard,Langford Lane Kidlington,Oxford,OX5 1GB,UK Registered Company Number 1982084 Customer name Adrian Wydra Customer address 688 University Ave West Windsor, ON N9A 5R5 License number 3159141172118 License date May 31, 2013 Licensed content publisher Elsevier Licensed content publication Journal of Biomechanics Licensed content title Cancellous bone biomechanics Licensed content author D.P. Fyhrie,J.H. Kimura Licensed content date November 1999 Licensed content volume number 32 Licensed content issue number 11 Number of pages 10 Start Page 1139 End Page 1148 Type of Use reuse in a thesis/dissertation
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Copyright © 2005 Lynnerup et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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VITA AUCTORIS
NAME: Adrian Wydra
PLACE OF BIRTH:
Kozienice, Mazowsze, Poland
YEAR OF BIRTH:
1985
EDUCATION:
Chalubinski's Secondary School Radom, Poland. 2000 - 2004 Wroclaw University of Technology, M.A.Sc. Wroclaw, Poland. 2004 - 2009 University of Windsor, M.Sc. Windsor, Ontario, Canada. 2011 - 2013
